Yep, have to agree with the Count and Tracker. Councils a full of people whose main priority is doing the minimun work and picking up their weekly "appearance money" each Friday. Very few know much about their jobs and if asked a hard question like "can I detect" then you will get the duck shove form dept to dept. As it's been said before here there is a 99.9% chance that a council by-law exists disallowing detecting type activities. Ask and they have to say NO!!
As most council employees don't want to create work for themselves I find that groundsmen (unless they are the head one) just get on with their mowing etc won't even bother you unless you are leaving unsightly holes in their lawns. When you are approached be nice and friendly (remember you are in the wrong) and if asked what you are doing say "poking about but mostly I'm cleaning up the park" I show them a large collection of rubbish and pull tabs I always carry just for such an occasion. I also carry a daggy old penny that's been hit by a mower and if asked whether I have found any coins show them this. I also dress like a council groundsman, tan wide brimmed hat, tan work shirt, tan shorts and dirt guards over my work boots. From a distance I look like a workman with a line-trimmer, also I think the pesky public are less likely to annoy you if you look like a council worker. I'm always curteous and friendly and to date I have never been asked to leave a park. In fact I have even been told by a groundsman a good spot to try.