OK, that's great. The sums I did indicated a 1mm skim would change a 10:1 to ~11:1

Here are the figures for my head/engine (measured with oil):
combustion chamber 45ccs
head gasket 0.63mm thick, volume 3.6ccs
piston crown volume 9ccs

Hence the total chamber volume at TDC 57.6ccs
From bore and stroke (not checked) the swept volume is 497.1ccs
Hence total volume at BDC 554.7ccs
CR=554.7:57.6 or 9.63:1 so doesn't quite make the factory 10:1

Remove 1mm is 5.1ccs ( due to the small 'crescents' on the head you get ~90% of the full bore area)

New CR 549.6:52.5 or 10.5:1

If the engine started at 10:1 the total chamber volume at TDC would be 55.2ccs (55ccs is what seems to be quoted for the Zetec head)

A 1mm skim would then give 547.2:50.1 or 10.9:1

I was a bit concerned about going for more than 1mm in one go and with stock pistons I think 11:1 must be absolute maximum, probably 10.5:1 is a 'safe' level.

Anyone got views on knocking, is it an issue at ~11:1?