it can be done easily by someone who knows wat they are doing but 1 wrong move and it will not work. you have to make sure no 'bubbles' are left putting the film on, and you cut carefully around the edge. not sure about the stuff in B&Q but there is a range of product called 3M which are excellent. There are several companies which specialise in this kind of thing but the only i can think of off hand is prolux
if you go to your local window company tou can get some off cuts of glass from them very cheap its not a big job you would be able to do it yourself. All you do is carfully remove the timber beads and take the pains out then using clear silicone replace it with whatever you want.When measuring the the new glass take the internal measurment once you have removed the timber beads.
A tip I heard on Changing Rooms was to wipe over the glass with washing up liquid before you apply the transfer, this allows you to manouevre it a bit more easily. Another one was to protect the wood and then use an etching spray. Never did either so I don't know how good these tips are!