If you have to wait for the T6 then:
* the new excise duties will add several £k over the first 5/6 years (see other thread) into the equation
* if you are selling yours privately, and can hold on 6 weeks, spring will be in the air and demand will be high. If you wait till the T6 arrives, Autumn will be in the air, and demand will be tailing off.
For someone without a van, it's close to a no brainer to buy the T5 and enjoy in straight away, but as you already have one, that doesn't impact you.
There has never been more than a couple of £k difference between used T5s and T5.1s. As the T6 is no bigger a jump, it should really be considered as a T5. 2 and I expect the same sort of differences to apply after a few years.
If they take your bid, you are certainly on the right side of the bargain.