Odd, we must be weird. First thing we did was speak to a mortgage advisor, find out what deals were out there, what we could borrow, repayments etc… before we even looked at houses. In fact we got our mortgage agreed in principle before we made an offer. Surprised you had your offer accepted being in this position. I certainly wouldn’t take my property off the market if the people offering didn’t have a mortgage agreed in principle.
If you have or expect to have a reasonable amount of cash hanging about an offset might be a good idea, otherwise a repayment is generally recommended if you can afford it. We are lucky and pay something silly like 1% over BR at the moment. Think if you are mortgaging now 4%apr is a decent rate.
EDIT – of course you might not be in England or Wales so things may be different…