Toby;

"Close fitting ring around the perimiter of the piston"...do you mean the ring at the edge of the head gasket, or the top comp. ring? ...in any case, when compressing the charge, is it not forced into the entire combustion chamber volume, by definition (pressure is the same everywhere in a pressurized vessel - dynamic flow effects notwithstanding)...

...and what about this "Squish" thing??...I did a search and came up with an effective way of measuring it (using solder) on Howard Richards site:
http://www.mix-net.net/~red/index.html
(an interesting, informative site, with more especially good info on determining Compression Ratio - I emailed him, maybe he will chime in here)
...OK, so I can measure it...but I would still like someone to explain its importance, or even just significance to me...

Cheers