ENBC (English National Bus Card) offers 34% off Westbury to Weymouth (and intermediate) off-peak fares ... so here is my advise to the seniors group in Melksham:
Members of your group may be interested to learn that their English National Bus Concession cards ("bus passes") entitle them to a 34% discount on train fares from Westbury to Dorchester and Weymouth, and from Swindon to Cheltenham Spa and Gloucester (but not, I'm afraid, between Swindon and Westbury).
So - Melksham to Westbury - ^3.50 day return. Westbury to Weymouth via Yeovil - ^17.00 day return - 34% = ^11.20 ^. total price ^14.70
If three or more travel together, they can ask for a "Groupsave" instead - and it will then cost just ^10.90 return all the way per person in the group, and that price also applies to adults in the group who don't yet have a concession card.
If you have contacts / members in Chippenham, they can travel for ^16.50 (one or two) or for ^10.90 (groups of three or more) to Weymouth without using the split ticket / ENBC card; in their case, the saving made is outweighed by the higher cost of splitting.
Members travelling from Trowbridge to Weymouth can also split at Westbury, again for a total of ^14.70 for one or two. Groups of three or more should book Melksham to Weymouth Groupsave tickets at ^10.90 even if travelling from Trowbridge, as a Trowbridge to Weymouth Groupsave is slightly more expensive and people may buy a longer ticket and use it 'short'.
The main advise is Melksham - I have added the Chippenham and Trowbridge advise for out of town members who may prefer that railhead.
Question ... can the TransWilts Conductor sell the ENBC discounted Heart of Wessex tickets to the seniors at the same time as (s)he sells them their Melksham - Westbury tickets, or are these tickets only purchasable at Heart of Wessex outlets such as at Westbury station.