For tunnels think the oldest long wings which used to have wards but now are mostly empty. these are attached to the ballroom and slightly behind the chapel. Ignore the really brand new tower developed in front of the old asylum.
- Easiest and best prospect would be W and especially SW of ballroom (some signs nearby for k block) where renovations are happening now and builder fences are left open often at night. If you go to lowest floor once in the areas where they've removed windows and are doing renovating work, might be easiest place to find possible doors to tunnels. Esp while windows are removed.
Just look for long building attached to SW of ballroom, you can spot the long wing with windows missing at the south side where access is easiest. the site seemed to be left open where there's a makeshift fence up by the builders when I went recently. you can just walk (20 sec walk south of chapel/ballroom) or drive to here behind the old part of St Bernard hospital. no road barriers as you walk (or drive but then where would you park...) along the internal road behind the current old building. they are renovating it, all just south of the chapel. easily accessible because lots of gaps where builders are doing their stuff and leave fences open and windows missing. this would be the best prospect.
- different tunnel areas are all underneath the overall big long building (west of which is being rebuilt and worth trying to get access) which then attaches to ballroom and then to chapel. you can see some steps outside at various parts which hint at other doors to the tunnels, they seem to head down into the ground.
- Some of the SE areas are in use so harder for access. There's a harder place just SE of ballroom, behind the building to which ballroom/chapel is attached. There's one locked gate about a metre high (maybe less) by the car park behind which is nothing but steps right into the ground, steps start at ground level and go down so v likely to a tunnel. curious! i think it's behind student area (signposted student/E block) with obvious steps downwards into the ground (can't remember if gate could be jumped over?, ive arrowed it on the map)
Might need a recce or two.
Annotated map here - north is top: