All rough calcs but
If OP is mid 40s she probably entered the pension scheme early enough for pension to be paid at 60.
The 10,000 will be the amount you would get at 60 if you stopped now.
I don't think it will go up anything like 5,000 a year, but something more like another 15,000 should accrue if she works until 65. Or if she has had a rise in wages / time out, a bit more should build up from the current sum.
Ex can't be entitled to your future earnings. So it is about half of the £10,000, balanced off to deduct whatever is left of his own pension.
Back of envelope he might be due 100,000 but that could drop depending on his pension holdings. So whether it's worth you splitting the pension or doing it with equity depends a lot on the overall value of the house and your future plans.
If you split the pension and keep housing security you should have about 25.000 a year from 65, 32.000 from 67 (state pension).
If you're likely to downsize I'd look at doing that now and preserving more pension but ex may well be entitled to less than he thinks, especially if he's assuming a share in the final amount.