Words of advice? Sure, these are the things I did, and only one larva has died and that was because I squished it with a net.
bare bottom tank.
I used a 2.5 gallon for 20 larvae untill size differences became too far to ignore.
I fed a diet of nothing but diced, chopped, then whole blackworms, according to size.
50% water changes every week, the water doesnt get dirty because the blackworms are alive till theyre eaten.
deep, cool water.
and if youre going to plant the tank go fake, so you dont need substrate that the worms can be lost in.
yep, with that you can expect inch long, healthy, fat larvae in 2 or 3 weeks, like i have right now. hope I helped ;]
PS. Since you live in the UK i doubt blackworms will be available but i beleive you can also use chopped bloodworms and tubifex worms. these are not as nutritious as blackworms though, so try and get some as soon as possible.