Am I correct that extending the ringmain is required to be done buy someone qualified?
No - it just needs to be done by someone competent. And there's no difference in the legal requirements or the level of competence needed to add an FCU.
I do not want to add more sockets onto the wall because it will look odd if I choose to sell the property.
If you installed an FCU, have you thought about where you would put it and how you would run the supply cable(s) to it, and from where, and how you would run the load cable from it to the sockets?
If the fish tank is against the wall, why not just have surface mounted sockets, maybe in a length of dado trunking, which could easily be removed if you sell?
The manufacturers instructions for metal halide lighting advise plugging straight into the ringmain due to high start up load (I cannot remember specifics).
Are you currently using these lights with no problems? If not, i.e. it's a new upgrade to your system then you're going to have to have additional sockets on the circuit whether you like it or not.
The idea of sockets under the tank is more to do with neatness although arguablly can be achieved with switchable extension leads.
You should probably start looking at suppliers of perimeter/dado trunking, find a metal product which you can spray to match the stands, and fix it to them.
Or - these didn't sell:
http://cgi.ebay.co.uk/MK-Power-Pole...3QQcmdZViewItemQQssPageNameZRSSABASRCHAGBA101 - it might be worth contacting the seller to see of he still has them. I know Birmingham isn't that handy for Wiltshire, but it was a pretty good price, if you think you could incorporate them into the stands. It's a current product, so you can get specs off the MK website.
I thought it would just be neat and tidy to have a single supply to the tank via a fused spur and numerous sockets
I still don't see how numerous sockets from a fused spur is any easier to install, or would look less odd when you sell, than sockets on the ring. And if you are talking about sockets fixed to the stands then those are just extension leads by another name.
Other than plugging the light into the double socket, I have limited supply of sockets in that room and so it would be useful to have a spare should I decide to plug a.n.other item in rather than reaching under my tank to say, plug in the hoover.
So some more permanent sockets wouldn't go amiss anyway.