If you haven't been in that job very long, I'd say that they've put you on basic rate tax (ie, no tax free allowance).
If you ring the HMRC employee helpline (they actually answer, and are actually helpful, unlike most government helplines), they will be able to tell you what your tax code is. If it's BR, you're on what used to be called "emergency tax" and you can ask them to issue a notice of coding to your employer.
Just crunched some numbers, and after your £800 pension contribution, tax at 20% on all of it, and NI, it comes to around £1950.