>> I thought ac3 6ch WAS 5.1 (5 being the front left & right, centre, left & right rear - .1 being the subwoofer) <<
The 5 in 5.1 designates the 5 major channels that you've detailed as above and the .1 designates the sub.
>> hence the 6 channels, and that there is no rear centre speaker. <<
Nope, not quite. It's only when that centre-rear speaker's brought into play does it become a full 6 channel system. It also depends on the particular disc that you're playing. You can be set up for ac3 6ch, but if the dvd you're playing isn't encoded for 6 .. you'll just get trad. 5.1.
>> Isn't the rear centre speaker used on 6.1 systems (ie 7 channel)? <<
Yes, but even though there are 7 speakers .. it's still designated ac3 6ch.
I think it's Yamaha who started to confuse things before 6 was brought out by introducing 7 speakers into the traditional 5.1 setup.