how do you navigate IFR (legally) outside CAS in e.g. the Gloucester/Shobdon/Welshpool area
Compass and stopwatch old boy.
I don't think there's any requirement within the rules for enroute IFR (off airways at least) to use any ground based navigational aids. It always helps to know your position of course but its not a requirement as such.
Give me a GPS any day.