Fortunately, St Ives lock was ahead of us and the opportunity to put a CRT windlass to the test. Even more fortunate, the River Inspector was there, with a Great Ouse/Middle Level windlass, to a) work the lock for us b) to have the windlass conversation with and c) to talk about the swinging gate at St Neots. He is top man.
Following the test, I can now declare, that both varieties windlass will work on all Great Ouse manual locks.
As to the Middke Levels, I hear that Gayton Marina have said that the CRT windlass will now work on all Anglian waterways. Two years ago, Gayton Marina sold us an EA key. There was a fault in a new batch of keys. The River Inspector told us Gayton knew that. In fact he had been to the Marina to replace them...... immediately before being called out by us when our key wouldn't work! Call me picky but I consider them to be an unreliable source of information.
In the search for completeness, I spoke to the lock keeper at Stanground. She confirms that if you only own a CRT windlass youWILL need a Great Ouse/Middle Level windlass for the Middle Levels and that she sells them at £12.
Having had the opportunity to test the CRT windlass at a couple more lock I will revert to the Gt Ouse/Middle Level version. Although it is heavier and doesn't fit my holster the snug fit as you use it makes it so much more secure to operate.