I'd echo the comments of 30k+ for those skills/experience, and then load a bit on for London weighting.
When I got my current job about 2.5 years ago I had no qualifications, just 4 and a half years previous experience. I walked into the job pretty much (I spent some time contracting there for a month first) and was on more than the 30-35k bracket, plus lots of other niceties (good holiday, 'green travel' allowance).
That's a pretty basic 3rd line server support team role, being the 'specialist' in a couple of areas
My salary has gone up since being there, and whilst I've since done a few certifications (including some crappy itil stuff, cissp) they've had no bearing on my salary. I'd get paid the same if I'd not got them, but it's been nice to get all the training I've had paid for. I reckon over 2 years that's 7 grand of training alone.
My problem is I wanna stay living down here really which is limiting me, could get some juicy contracts or just a better paid perm job if I were to move nearer London.
Our desktop guys start on a trainee wage of about 21k, moving up to 30k over a 3 year period if they do their exams.
Out of interest is Oxford too far away for you? I've got details of a job paying up to 45k there, I'm not interested as it's too far but the recruitment guy said if there's anyone else I know who'd be interested to pass the spec over to.