Who's a silly billy then!! When I referred to the chambers on the level above, I should have realised that these are what everybody calls the 'pits above the track'. However, they are genuine interconnected chambers, but completely opened out to bank.
I've just been back up there for another look. The adit which breaks into the top of the big chamber is easily found just to the left of the four pits (looking up from the track) at about the same level as the pit mouth. It's about 120ft long with a kink half-way & slopes gently down from the adit portal, hence the water draining into the chamber. There is a rather old 8mm bolt with a 'Troll' hanger about 15ft back from the drop into the chamber. In addition, there are two old iron bars driven into the wall close to the drop which look a lot safer than the bolt should anyone want to do some SRT practice.
According to the Conglog book, the adit leading into the pits is 'run in'. In fact it's open right up to the pit but maybe impassable due to fall within the pit (The water in the adit was over CWD & I hadn't taken the waders this time). If the adit is no-go, it should be an easy drop into the end pit on the right.