I have not had the belt covers off, but that is a good shout.
I am going to play a little more this morning. The plan initially was to remove the manifold to take a look, but had the customer on the phone to say hold fire because she is getting trading standards onto the selling dealer ( even though he bought it 14 months ago )
Regardless, I am reluctant to let this go without a fix so am going to carry out some more tests and post up some actual values.