Found out that I had 2 voicemails from my manager while I was wondering around the Palace. One was asking me to call him, and the other asked me to call the IT head-person.
So I rang up the IT person and he said that between HR and IT they did make a mistake, and as such they are going to offer me a base of GBP16,000. Then (and I assume it works like this) ontop of that there should be the Outer London Allowance.
In the end I should be getting just under GBP18,000 which will be brilliant. Waiting for a new letter of offer to come through, but I will just hand in my acceptance letter to the HR lady on Monday morning.
After having been here for 10 months or so it would seem that things are finally starting to sort themselves out.