This:Which part do you believe is incorrect?
In the small claims track the costs that can be awarded are minimal (comparable to their "admin fee") and the amount they can be awarded is only the loss which is very unlikely to be more than what they've offered.otherwise it will end up costing you even more than they are offering to begin with.
This is not the usual "they have all the cards" situation, here there is a *lot* of room for OP to negotiate and there is very little risk to doing so since letting it go to court won't increase the cost above what it would cost to settle now.
The only exception to that is if the OP is ruled to have behaved unreasonably, so they should come back here if/when GWR reject their counteroffer and send a letter of claim so they can respond appropriately. Capitulating now has no benefit other than paying GWR a bunch of extra money to avoid having to write a few short letters.