If you're in Essex, you might also want to check out Charterhouse Aquatics. I've not been, unlike STM, but it's got a good reputation I think. Whereabouts in Essex, btw? When I was still at school, I lived for 9 years up in a small village between Bishops Stortford and Saffron Walden.
http://www.charterhouse-aquatics.co...idx=1&osCsid=1c51d165fb8b9782e4d4aded408caeb8
Full credit to you if you want to go the DIY route. I could never face it, much preferring to have a set up that's designed to do the job. It tends to look a bit smoother, as you don't have to cut bits out, glue things together etc, but you're right there's probably not the same sense of achievement. The big challenge with the DIY route is drilling the glass for an overflow, but if you're confident about doing that and in working with silicon, you should be fine if you do your research (I don't know any of the details of how to do it, but plenty of people do, so it must work!). Plus, as you say, you'll have it done to your own specs and, unlike me, your DIY will probably not look messy, so you don't have that to worry about either.
The reason I recommended the Vortechs btw, is that they produce powerful yet diffuse flow which is ideal for a reef tank. They're probably the best product on the market today, with their nearest competitors being Tunze Streams/Nanostreams (very slightly more reliable, but unlike the Vortechs, they're not wireless and have a bigger profile in the tank plus fewer options for controlling the flow). I have Tunze Nanostreams, but that's mainly because I ordered them from overseas (the US) and wouldn't have the option of returning them if something went wrong (both products are high quality btw, it's probably only a minor difference in reliablity - maybe between a 1% and 0.5% failure rate, but not being in the same country as the shop I was buying from I decided to try and reduce my risk where I could). From what I know of anemones (and that's not a huge amount admittedly), they do well with this sort of diffuse flow (though they won't need quite the same power SPS corals do, the Vortechs and Streams are adjustable).