Seems that without a subscription I am still limited to 24 groups but if I subscribed I could have 64 - which they reckon is 39 more than I can have at the moment. Is it my maths that are wrong? I wonder what would happen if I subscribed, created all those groups and then let the subscription lapse, would I lose a load of groups?

It seems the setup is you get it with an Ancestry subscription of any kind. It's also saying I have 25 groups, but I only have 24.
Hopefully they just don't let you edit once you are up to 64 groups and your subscription expires.
To someone who mentioned about 32 colours. The setup is you have 32 colours and then each group has a letter to differentiated.
This has been a much needed feature for some time. Using the old limited DNA interface I grouped all my matches down 20cM. Since you could not see shared matches below that. It's just a shame Ancestry feel the need to bleed their most active customers for what are minor development updates. They really should be providing a lot more for £7.50/$10 a month:
- a chromosome browser
- find matches by shared segments via the browser
- show common ancestors in all your DNA matches trees (including extrapolations of those trees)
- a tool that will auto-group clusters of matches (including low cM matches)
- more advanced tools for cataloging the matches; at the moment I use a spreadsheet, but if you could use a system built in to the website it would be much more powerful, letting you more clearly see who in which clusters are DNA matches
It would be worth the money at that price range.
24 groups were not enough. I have about 10 "overflow" groups down to 20cM+ groups for each of my tests. I have designated those by adding notes like: #OVERFLOW - Maternal - Welsh654#, but it's more cumbersome to work with those.