It looks deceptively small from the outside, but that's a trick of the landscaping. Old Mill has a big Tudor-style dining/dance hall, and it's clear why it's popular for weddings and other parites. They've got live Jazz some nights, but steer clear of the cover band, unless you want to see a lot of retirees shimmying to bad Beach Boys renditions.
I was last here for Summerlicious, and the meal (lamb with spicy corn and beans) and service were quite good. They have a dress code (jacket and tie), but it doesn't seem to be enforced very strongly--which made me feel a bit silly when I was there looking all spiffy.