Thanks for all your comments

I would never dream of coming into a company and earning the same as the existing staff, however, I have always worked in companies where the job is worth £x and there is a grade to follow upon performance/service length etc. The previous PA started this role 4 years ago on £xk, and progressed £3k in 4 years. I started a year ago on £1k less than her starting salary 4 years ago. The job hasn't been devalued in any way in fact it's been added to as the years have passed by all accounts! I guess this is my main gripe!
I would have negotiated a better deal but was told at interview the salary we are offering is not up for negotiation and as I was better off I was happy to accept, but now feel that I've screwed over (for want of a better term).
In fact the reason I know what the other PA is on is because she told me and right now I feel very much like my face is being rubbed in dirt.
The fact that my boss has told me he can't offer me £4k isn't a big deal, as I've said, if I'd worked in the co for x no of years and a new person started on the same salary, that would upset me more. I just feel like my skills and qualifications are being taken advantage of and the value of the job isn't important, it's what/who you know!
I guess I need to brush up on my negotiation skills

Callie