My engine was 100% oil tight up to 46,000 miles when suddenly without any prior warning a very bad engine oil leak appeared from the bell housing area suggesting that the rear main seal has failed. I have checked the breather hoses and pcv valve and these appear to be clear and functioning corractly. Can anyone please suggest what may cause sudden failure of the rear main seal? (Assuming this is the leak source). One would expect a seal to start leaking gradually, giving warning of pending failure.

The seal might have been "trying" to fail and just had to wait for the right (wrong?) conditions to finally do so. Have it closely inspected to make certain it is actually the seal.
Definitely engine oil?

redrocket55 disagrees: Instant failure, whilst not impossible, is extremely unlikely
I wasn't voting for instant failure, only attempting to explain the conditions you described.
Good luck getting help next time, red