Limited budget - suggestions for upgrading my PC

Thanks for the comments, however l really needed to stay within my £500 budget (which I exceeded by £77!!).

Yes indeed, you make a very good point about there being a slippery slope, and as you go substantially higher, prices can balloon faster and faster. On the other hand, there are "sweet spots" in the price/performance curve, and I do indeed believe the 1600 is one of them (the 1300X and 1700 being the two others for different use cases,). Due to the economics of how these chips are produced, the 1600 (and even more so the 560/1050Ti) really is the inflection point above which price increases faster than performance, and below which performance drops as fast or faster than price due to fixed per-die costs.

If I were to argue for a single tweak, though (presuming lack of RX 560 availability), the 1600 would be it, and while again the SSD, RAM, and mobo you picked are all very solid choices, scrimping just a little in any one of them (particularly the SSD, even though I highly recommend the 960 Evo) to compensate should still leave you well ahead in current and future performance.

Ryzen can use 3200Mhz RAM, and whilst the Flare X listed on the same site I purchased the components from listed Flare X, they only have 2400Mhz speed in this series

I see, makes sense. I didn't even know Flare X went down that low; thought 2666 was the minimum for that series. As mentioned, this is at your build level by far the lowest priority, and my intention was more to suggest a potential future upgrade for yourself or others as opposed to a recommendation for now. In any case, their cheaper Fortis line at 2400 could have perhaps save a few quid over the Corsair model you bought, (that could be applied toward the 1600) but I imagine you probably already looked into those options.

As for the GPU, as you mentioned you would have purchased the 560 if you could, which would certainly have been the best option for your build, but due to the crytomining craze it was out of stock and likely would be for some time, so there wasn't much you could do without increasing your budget somewhat. What I might suggest, though, since your 550 seems to work well for you, is to hold onto it until 560 prices/availability stabilizes or Nvidia comes out with Volta and the 1050Ti goes on clearance, then sell the 550 and upgrade in due course. Combined with the 1600 or a Zen 2/3 equivalent down the road and the rest of your existing build, that should give you a very solid machine with some room to grow for many years to come.

But for now, it sounds like you have everything you need to produce great home content, which is what it's all about—not worrying too much about hardware specs like I tend to do!