I was trying to keep it not cheeky as it’s not shown on the os map as neither footpath nor bridleway.
That section of the South West Coast Path is MoD land, and so it is not a Public Right of Way (Bridleway or Footpath). On the 1:25,000 OS map it is designated a ‘Range Walk’. As I recall there is a fence and gate through which you have to pass, and various restrictions are on a sign by the gate, doubtless these will prohibit cycling.
could you tell me if the following route is rideable (i’m happy to walk a bit)
Even if it were permitted, and even if some have ridden it, I struggle to think of a worse route to cycle. Much of it is extremely steep, so not only will you probably have to push your bike up the hills, you will probably have to brake hard and continuously all the way down the other side (or even walk down). You will probably end up going no faster, and possibly even slower, than the walkers, some of whom may share with you their views on your choice of route and the legality/appropriateness of riding it, and because you will be going so slowly, you will not be able to get away from them.
I have a radical suggestion for you. Leave the bike at home and walk along the route. It is a truly stunningly beautiful bit of coast, and walking will allow you to enjoy it much more than trying to ride it.
The opening times are here (note it’s closed on the weekend of the 14th and 15th):
http://www.tynehamopc.org.uk/tyneham_opening_times.html