If System Restore is taking a snapshot of my computer every day or so the collection must build up fairly quickly! And these snapshots would surely become irrelevant after a period.
Is it possible to delete old SR points? Or would that involve some dangerous messing around?

You have the option to set the disk space allowed. From 0 --> 12% of your hard drive. 12% is default. When that is used the points begin to be dropped as new ones are added.

The settings are via right clicking My Computer then click Properties then System Restore.

Here you can also toggle off and on System Restore which will erase all points if you so desire.

Got it! Thanks.