Hiya, People assume that mice are easy to get rid of, they aren't that easy, unlike rats. in my view the biggest mistake was to block up a hole where the mice have clearly been evident, Closing up these hole usually compounds the issue by pushing mice into the cavity which means they have then to search to find other means of escaping the cavity. I would have kept a mouse bait station there permanent and kept it topped up with poison so the situation and infestation can be monitored.
Your pest controller should be putting specific "mouse poison" in your loft, dropping it down the cavity if that is possible from your loft. When you hear a mouse in the loft it is usually quite distinctive, it sounds very different to gulls walking on the roof, its more of a scurrying sound or distinctive scratching/knawing. He should also be putting down quite a few bait stations, mice are sporadic feeders and only take small amounts of food /poison at any one time so they need lots of bait stations to feed from. Personally I wouldn't touch Rentokill with a barge pole. They charge extortionate prices. You would better just ensuring you have a reputable pest control company registered with the BPCA and displaying their logo in advertising.
MissCommando, The pest control company should give you sister some assurance that their treatment is guaranteed to sort the problem and they should be coming back about once a fortnight to check the situation and to monitor the infestation until the problem is resolved. The Mice will be gone but I would expect it to take 2 or three weeks, if it were rats I would guarantee they be gone in two weeks but usually just a few days.
Brinjal, whilst rat poison will often kill off mice, rats and mice are different creatures with very different feeding habits, I have known mice eat all of buckets of rat bait with no ill effect, but as soon as they they touched the mouse bait it was "goodnight vienna!"