Thanks Gavin - I contacted Tim and the answer is :-
The difference between £3.88 and £5.00 is that for £3.88 you can effectively 'rent' the video and watch it online whilst logged in. The £5.00 fee is to actually buy the video; download it so that it is on your desktop.
If you subscribe as a monthly payment (£25), you have unlimited access to all the videos available, but this is just online when you are logged in for the duration of the month - with no limit to how many times you watch them in the month. It is only buy buying an individual video for £5.00 that you can download to your desktop with no time limit to watch.