Ah ha!
Greg
You could use Memory Map 2004 Ordnance Survey mapping software to mark the routes or waypoints and upload them to your GPS. It's just like using Mapsource but the mapping data is the OS Landranger series at 1:50k. That would save you riding around everywhere just to "mark" the end of the BOATs and RUPPs. The caveat with marking the maps up at home is that you wouldn't be aware of any TROs in force but you'd soon find that out whilst riding. Also, be careful of the RUPPs - these are the ones that are usually being downgraded. Keep an eye out for ORPAs, you can ride some of them.
Your local TRF group may use the same software and already have a dbase of trails that they can email to you.
Dave (KTM520) has the sofware..........
Here's a quick screen shot of MM2004 and a couple of marks. For the observant out there, it's the last trail we rode the other week, just before hitting the pub.