I've seen one pretty limited set of photos from someone who used a ladder that was there for a short period of time and got into Redoubt No.5. They're on another site (DP).
There's also a set of interiors of Renney Battery on Flickr HERE.
Drake's is currently open with tours operating, they didn't look well equipped though, and a massive group, so I'm not sure if they're doing the underground bits.
There's also Airsoft running once a month, playing on all parts of the site that have been cleared inclusive of tunnels, I grabbed what shots I could during play, but only had my phone and a WML, so they're far from ideal.
Few samples below whilst I'm waiting for the ferry.
Any one who enjoyed time at Junior Leaders Regiment Royal Armoured Corps will have memories of Rennie.
The left hand battery was a store for training kit and the other was used for drying tents and stuff.
Being forced to bathe in the sea in mid December off rennie rocks. Cold dark wet Nissan huts. I believe it’s private houses there now. There was a pillbox right at the entrance by the main road.