Welcome to the forum!

Have you got any marine salt and a refractometer?  You'll need to be gradually raising the SG using marine salt in the water changes.  Using plain RO isn't going to raise the salinity at all.

Algae on the glass needs to be physically scraped off.  What powerhead irculation pumps have you in the tank if any?  Algae tends to cling on where the flow is not too strong so maybe your circulation round the tank needs to be addressed.  The red cyano on the sand will need to be syphoned up and should go with good nitrate removers and the extra flow.  A good CUC will also help with eliminating problem algae too.

I wouldn't add any stock to the tank until you have got the SG to 1.024-1.025.  Then start with some CUC like turbo or astrea snails (the latter have pointy shells and need to be kept an eye on...if they fall over they can't right themselves and they die). Dwarf hermits like the redlegged hermit crab are good but it's best to get various spare shells of different shapes and sizes for them to house swap in and out of.

Weekly water changes with RO alone may not get the nitrates down that fast.  You need some form of chemical filtration like Rowaphos or Seachem Seagel/Purigen or such.
You can put chemical filter media in the internal box filter or (better yet) the external filter.  This will help keep on top of your water parameters.

You may want to look at some internal powerheads as I mentioned...Koralia pumps are good or there are Tunze Nanostream pumps which I hear have a better flow angle.  These will help keep the flow of water moving round the live rock inside the tank.
How much live rock do you have in the tank itself?  You need to allow roughly 1kg of live rock per 10 litres of water...so around 12kg would be ideal for your tank.

Any piccies of your tank?

Hope we can help you...there are a fair few slaty seadogs on this forum and maybe together we can get you back on track

Thank you so much for your help.
I have a water circulating pump (looks like a hairdryer?) positioned so that it intersects the flow from the internal and external filters, so the circulation is quite good. The corals seem to move quite a lot with it anyway.
I am using Rowaphos in the internal filter as I had a high Phos issue, this keeps the phos level perfect but will it help the nitrates or do I need something else?
It's hard to judge how much rock I have. I would say there is 12kg or more with all the stuff in the filters too but I was thinking about getting some more to add height anyway. I'm not bothered about the time it takes as we are saving to move house so I don't have much spare cash and think it will be less stressful to move with less critters anyway! I will weigh the rock when I move it all as it would be useful to know.
I scrape the front and sides daily, much to the disgust of the hawkfish, who likes sitting on the scraper! But I didn't know I could syphon the stuff off the sand without sucking all the sand up...pretty obvious now as I remember doing it in my tropical tank!
I will get some salt and add it as you suggest, jimthefish, at the moment I have to buy ready made salt RO as my big hairy dog makes it impossible to make up my own at home unless I sieve it first!
I think I am making progress as all my tiny little rock critters are back out and the rocks have a beautiful coat of pink again. My 2 long long tenticle leather corals look beautiful and are chucking out babies, so I think there is light at the end of this tunnel (my other leather is sulking as it fell of it's rock so it's currently being tied back on with elastic bands. Whole other problem though!) I realise my lighting is not sufficient for many corals, but I don't have plans to keep any others at the moment.
I will put a pic on, and resist the temptation to remove all the alge first so you can see what I mean!
Thanks again for your help!

My new camera won't connect to the computer, so unable to download pics. Will let you know how I get on!

Late response I know but..

I think depending on your desired SG its about 310g-450g per 10m litres  I measure for 310 and it gets roughly 1.024, I then add a bit more salt if needed

35grm/lt is the usual mix. Your refractometer should have two scales, one SG the other as a percentage or grm/lt figure. 32grm/lt=0.32%=SG1.024 approx.

Righty, been to see the poor guy in the fish shop again as although my Phos is ok, my nitrate is still sky high even after months and months of water changing. hHe's given me a 2 month 'drastic offensive', as he agrees this is getting stupid!
I have to remove the small bits of live rock from my filter units and just leave the Rowaphos and add a bit of plain filter wool, then do a 50% water change every week, testing as I go of cours. The first change I have to remove all the rock and swoosh them in a bucket to remove any 'clingy' nitrates. I aso have to test my new water before I put it in the tank. His theory is that I'm either adding the nitrate in the new water or it's being produced by the rock in the filter compartment and 'stored' in the tank rock.
As he has all kinds of fishy degrees and qualifications, I'm going to follow it and see where we end up!

All sorts of fishy degrees? Reminds me of child care social workers who have no children somehow. I may be doing the guy a great dis-service however, hopefully he'll get you sorted. Look forward to a progress report. Are you now making your own? If so are you using RO water? Tap water could be a source of nitrates.

sounds like you are getting grips with things.  I had to laugh at the line about the dog hair, I have to prevent a toddler putting their hands in the bucket