Hi Mervyn,
I recognise that sort of algae. And I agree it is very frustrating. I had the same issues as you in that it was smothering my plants. And it isn't one that is easily identified from the list of James' planted tank site. I think the one it most looks like is the brown aglae or diatoms, but I'm not totally convinced that it is this.
How long has your tank been set up? (Several months, based on my memory of your other posts). What water test kits do you have? If you have a full suite of test kits, it would be really helpful to see what readings you have for things like iron, calcium, phosphate and nitrates, general and carbonate hardness (GH and KH).
I'm desperately trying to remember in what order I made changes to my tank to solve the various different problems - the first was the annoying algae. (Once I got that sorted I found that some of my plants weren't growing very well and that was due to other mineral deficiencies, which when tackled more or less obliterated the remaining algae issues). On the old board, I had a lot of great conversations with Natalia, but unfortunately my very pregnant brain is not functioning too well at the moment so you'll probably need Natalia to remember our discussions, when she has a chance to post!
Do I remember you saying that you've added the T5 lights as a replacement for T8s? If so, what was the wattage of the T8 tubes, and I am assuming that you changed the ballast as well. T5s produce a lot more light energy than T8s (for the same tube length), so if you have upgraded, you will have increased the light energy available in your tank. Also T5s are available as HO (high output) which are more efficient and will give you even more light energy, do you have these?
Optimising plant growth and minimising algae is all about balance. So if everything is in balance, and then you increase the light, you will increase the algae issues as the plants can only grow as much as the least available resource. All the algae is doing is mopping up the excess of the other resources. I spent a long time measuring my phosphate levels in my tank and agonising over why a reading of about 1ppm (which is fine for a planted tank) would be causing me problems. Then I read in an article that the ratio of phosphate to nitrate was the important factor, not necessarily the reading of the phosphate. This is when I realised that a lot of my algae problems were due to very low nitrates. So now I dose nitrates.
You've already said that your nitrates are much higher than mine ever have been, so this is unlikely to be the cause of your algae issues. But by understanding the ratios of the other minerals and nutrients in your water, it'll be easier to work out what needs re-balancing to get rid of the algae.
Have you tried having your lights on with a siesta period? If you do, you'll probably need to do this for a couple weeks to see if it has any effect and have the lights off for about 4 hours in the middle of the day. You could have them on for about 5 hours either side, if you wanted.
Also does the low winter sun catch your tank at all? I moved my tank in the spring, only on to a perpendicular wall, and this made a big difference to the colour of the water (which had always had a very feint green tinge only visible in the white bucket I use for water changes). The move meant the tank was totally out of reach of the direct sunlight. Until the last few weeks in November. And the vague green tint has come back to my water, but not as much and it doesn't cause any other problems. So I've decided not to do anything significant about it (other than slightly larger water changes just to keep control of the algae) as the sun will soon be a bit higher and no longer reach my tank (in the mean time I try to keep the curtains closed in the mornings!)
It might also be worth having a look through the Tropica website to try and identify your plants. A lot of the UK shops get their plants from Tropica, and their list of plants is pretty comprehensive. It also tells you what basic conditions each of the plants like, so you can identify whether you have all low light plants or a mix (I have a feeling that red camboba is a demanding plant).
Sorry I haven't been able to give you an answer to your algae problem, but I hope I have given you some ideas to try out. Keep persevering and it will sort itself out. For me it was a very sudden realisation that I'd not had to cut nasty leaves off or scrub the front of the tank for ages, normal maintenance was enough to keep it looking good.
Good luck.