If
@Nikki is still following the thread, it's perhaps worth summarising where we are:
(i) we'd be able to give more focussed advice if we knew a bit more about the tickets in question
(ii) but without knowing that, it's worth pointing out that for your claims to refunds to be valid, they will have needed to be true i.e. that as you told Trainline, you did not make the return journey
(iii) your claim also needs to have been totally accurate - that is, if you quoted an amount of money that you wanted refunded, it was correct by the railway's rules. For tickets described as 'return' tickets, that means (cost of the return ticket) - (cost of a single ticket). There can also be a charge of £10 for each ticket refunded.
(iv) If your claims met points (ii) and (iii), then you have done nothing wrong
(v) if you did not say how much you were asking to be repaid and Trainline gave you more money than they should have done, then that is a problem between you and Trainline, and nothing to do with West Midlands Trains. My view would be that realistically Trainline would not be trying to pursue you for overpayment because of their mistake when (in the great scheme of things - compared to the cost of houses for example) the amount involved is quite small
(vi) if (point (iv)) you have done nothing wrong or (point(v)) you have done nothing which is anything to do with West Midland Trains, the consensus advice is not to respond to the email.
(vii) On the other hand, if you are in the wrong, my view is that you might do well to try to settle this matter as quickly (and cheaply) as possible. If that's the case, respond to the specific questions that West Midlands Trains have asked briefly and honestly, and see if they will let you settle this matter out of court.