First thoughts on splitting the fares:
The Off-Peak Return fare from Chepstow to Gleneagles is £171.20, so that's what I'm trying to better.
I've used the dates of 24 October outbound and 27th October, providing just a short break, as my examples.
Splitting the journey into three ('Advance') fare sections (Chepstow to Birmingham, Birmingham to Edinburgh and Edinburgh) suggest the following:
Depart Chepstow 1018
Arrive Birmingham NS 1145
(Direct) £10.30
Depart Birmingham NS 1230
Arrive Edinburgh 1721
(Change at York, Arr 1439, Dep 1454) £53.20
Depart Edinburgh 1733
Arrive Gleneagles 1902
(Change at Stirling, Arrive 1829, Depart 1842) £8.40
____________________________________________
Depart Gleneagles 1253
Arrive Edinburgh 1437
(Change at Stirling, Arrive 1312, Depart 1337) £8.40
Depart Edinburgh 1451
Arrive Birmingham NS 1905
(Direct) £26.00
Depart Birmingham NS 1946
Arrive Chepstow 2149
(Direct) £10.30
So that gives a total fare of £116.60, which is 32% cheaper than the Off-Peak Return fare (and thus compares favourably with the 34% discount that Railcard users receive).
Give me some more time though and I might be able to better that!