In terms of through tickets (and applying your railcard discount to all fares mentioned in this post), there's an £86 offpeak return (not via London) valid outward on the 0730 arriving 1312 and returning on the 1628 which gets back at 2135.
There are Bristol TM-Shildon advances: £44.65 on the 0530 via London arriving 11.12 (this figure includes a discount, usually about 2%, for booking via the East Coast website); or £45.20 on the 0730 (Cross Country via Birmingham, change at Darlington) arriving 1312.
Coming back, there's a Shildon-Bristol advance of £28.70 on the 1628 arriving at 2135 (change at Darlington).
Travelling via Birmingham, you may save money by splitting your tickets as follows on the 0730 changing at Darlington and returning on the 1628 changing at Darlington onto the 1705 to Bristol:
Bristol TM-Cheltenham £5.80 offpeak day return
Cheltenham-Birmingham New Street 2 x £4.45 advances
Birmingham-Derby £3.75 advance
Derby-Birmingham £2.70 advance
Derby-Sheffield £7.35 offpeak day return
Sheffield-York £6.15 advance
York-Sheffield £3.45 advance
York-Darlington £3.75 advance
Darlington-York £4.75 advance
Darlington-Shildon £2.45 anytime day return
Total: £49.05
If you travelled on the 0615 from Bristol via Birmingham you would get to Darlington at 1113. This would save £1.05 (on the Birmingham-Derby advance) but you would still have to wait for the 1254 to Shildon unless you walked to Darlington town centre (Tubwell Row) and gained time at Shildon by getting a bus.
