I have a (very) small occupational pension with my previous employer, I wanted to surrender it but they said no, I either have to sit it out or tranfer to another pension. Can they do this? Surely it's my money?
No it isn't your money. It is money that has been put aside to provide a pension income for you when you retire.
In some cases you may be able to withdraw up to 25% of the cash value, but you must be over 50 and the scheme rules must allow it. But if you do this, it reduces the amount of any eventual pension.
If the scheme provider says you can't do it, it's probably because one or both of those provisos are not met.