hey dude i know it was proberly a lot of messing about putting that tin foil up but it's not good it causes heat spots and doesnt really reflect light good..If the wall behind the tin foil is white take it down dude the white reflection is better than tin foil.(that is just my opinion bro).
Your new light should be good if its around 50 quid you shout make sure its a "high pressure sodium" or "metal hallide" ( sorry if thats spelled wrong im dumb).
one more thing make sure you have some sort of ventelation like a little fan blowing the air around in the cupboard and something to get the heat out ( you could proberly leave the door open but if your new light is a HPS it will be really bright and HOT. One other guy said get some CFL's (compact florescent bulbs) there really cheap from argos and ****, they will grow much better than normal bulbs.
Oh..the gap under the door wont really be good beacuse you need it to be really dark, all i did was put a big bath towel over the gap in my door.
sorry, that was a lot of reading..If you need any more help let me know bro..
RAD