Well I rang up CS today and had some more progress... The representative actually had a clue what I was talking about!!
She said that I can't have my STB (I have an old Pace one) upgraded until April 4th and to phone back to make an appointment around March 21st.
I asked her if I can have my speed upgraded to 1mb in the mean time and she said "It's a bit risky 'cos the boxes are only rated up to 1mbit" So I said "do you fancy risking it?" and she did.
So I went from 750K to 1Mbit, and hopefully doubling it in April