Do you use any kind of GPS? I've got a Garmin Edge 200 (not sure if they are still available). It does not have maps in it, but it does have the facility to put a route in and it gives you a 'breadcrumb trail' - the screen shows you upcoming turns etc. It cost £60 last year. You could put in each day's route and use it to double check the sustrans maps - cheaper than buying OS maps for the whole route.
If you're worried about losing the route, you can just use your smartphone (if you've got one) to relocate back onto the route.
I've used sustrans maps for other routes and they are great - clear, and with notes about dangerous crossings, which bits are not on tarmac etc (not sure if this applies to the WotR), esp when there are a couple of options for part of the route.