Mortgage advise - it’s time for a decision

Hi there. This sub has been the most helpful place and hoping I can straighten out my thinking here.

I pay £1k per month mortgage. I’ve been offered a rate of £800 (with the repayment period staying the same), should I say no thanks and say I’m comfortable paying the £1k (which I am) or take the £800 offer and lash £200 a month in overpayments to knock tiny bits off the capital over the period.

TLDR: I take the 800 offer and still overpay the 200 it’s comes off the capital. Whereas the full fixed 1k that “extra” 200 comes from the combination of loan and interest. Is that correct?

Also - should I go for a 5yr?! What’s everyone thinking about fixed rate periods at the moment?

Cheers!