2 yr fixed is up in September, I haven't overpaid the mortgage this year, so can I overpay 10 % now and another 10% before the year ends as I'll be on a new 2 year fixed (or split)?
Sounds like you're with Ulster. If so then you can pay 10% of the outstanding fixed mortgage each year. I see no problem with your plan. If I were you I'd pay that first 10% straight away. As for the remaining 10% funds you have:
check what the cost of breaking would be now. The sooner you overpay the greater the benefit. Given the low term remaining it can't hurt to see what they say.
If it's not worth paying it off now, rather than re-fix all of your mortgage in September just fix that portion you can't over pay (i.e., 90% of September balance) then you can pay off the 10% variable straight away and still have the flexibility of overpaying a further 10% of the fixed portion if you need it.
General rule is you'll be better off drop feeding your mortgage overpayment rather then waiting for a big bang overpayment. If course the ability to do either will depend on personal circumstances.
check what the cost of breaking would be now. The sooner you overpay the greater the benefit. Given the low term remaining it can't hurt to see what they say.
The 10% per annum will reset. Pre-September and post September are effectively different loans - in break calculations at least. While the rates and term might be the same they would have different start dates. This would be needed for calculating things like break fees - September being the new reference date.
I would normally do it this way but just bought a car so need to build back up my emergency fund first (my wife still has her fund so we wouldn't be in trouble).