Stevenage? Hmm... GlaxoSmithKline springs to mind?
35k is a good salary, but it depends on experience you have etc and what your field of work is like. Salaries are on average higher in London because it is an expensive place to live in. 35k is certainly enough for life in London (if you are single) with ability to save, however I thought appartments (assuming again, for just you) would be cheaper than that in Stevenage.
If you plan to go to Lodon often, train tickets can be quite expensive, check the railway website for more info. Stevenage has direct link to Kings Cross and Finsbury Park. From what I heard, the place is very small and there is not much going on in terms of night life/entertainment. It's also advisable to have a car, because thre is no underground like in London and doing the weekly shop or going places will prove a hassle.
This is from other people who work and live in Stevenega, I personally don't so don't take my word for it.
Hope it helps.