Thanks all - I have the advantix thingy on my windows virtual machine but I'm clearly not intelligent enough to work out how to use it!
My method for working out splits is this:
1. Find an old envelope or scrap of paper which is reasonably large
2. Calculate the direct fare
3. List all major intermediate stations on your journey
4. Start by choosing one of these stations and calculate fares either side
5. If cheaper, now choose another station from either side and split further (remember to write down all the fares you find, to save looking them up again)
6. If it isn't cheaper, keep trying intermediate stations until something crops up that appears cheaper.
7. Golden rule: check the restrictions on the tickets to make sure they are actually valid on the services you wish to use
Remember also that sometimes it can be cheaper to buy a ticket for further than you wish to travel, provided a break of journey is permitted on that ticket (e.g. the SVR ticket on Bristol
TM▸ to Cosham, where Bristol TM - Havant is cheaper).