I have a Taunton to Durrington route Salisbury off peak rettun ticket but want to take bike too. There are no bike spaces available from Westbury to Taunton at any time when I want to travel. I'm thinking of going via Bristol TM. Is the ticket valid via Bristol if not what excess ticket would I need
No - it is only valid via Bristol if you travel via Swindon and Reading, for which a more expensive 'not via London' or '+Any Permitted' fare is required.
In theory the excess would simply be the difference to the cheapest combination of tickets valid for travelling this way, which would be (assuming you aren't making a day trip) Off-Peak Returns split at Bradford on Avon. That would cost £66.60, not a great deal more than the £56.60 Off-Peak Return you hold.
However whilst on the one hand I think it's unlikely that anyone would have an issue with you travelling via Bristol (it's hardly a massively circuitous route and you have good reason for doing so), if someone did 'pull you up', they may claim you need to buy a brand new ticket for part of your journey, e.g. Taunton to Bristol or Bristol to Westbury.
So to avoid potential conflict and overcharging it may be better to get a refund on your existing ticket and then buy the above combination anew. You should be able to do this fee-free under the Book with Confidence policy.