TWC - "30 hours surf time" might be a little misleading... it's surely down to what you surf - take a look at YouTube and it might cut the 30 hours down quite a bit.
Not sure what the 15 pounds buys, but if it is 3 GB it sounds reasonable.
Many of the top-ups last only 30 days on a "use it or lose it" basis (like some monthly contract for mobiles) which seems unfair if you don't need to use it more than for a couple of days.
On a separate thread, someone paid 79 (?) pounds for a dongle which included 12 GB of data and lasts 12 months so far less pressure than the 30 day system, and as you can monitor how much is left, can get a good idea of what you can do and how much data goes over the link.
Suggest you turn OFF any automatic Windows Updates, and certainly DO NOT watch things on iPlayer / 4oD / Five.tv as a single 30 minute TV programme might use 300-350 MB (and 3 of those will therefore wipe out 1 GB of traffic)