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we have tickets for St Martin-in-the-fields
Trying to do "event dinner" before a concert is probably trying to do too much. Avoid that temptation, even though you might not have seen the folks for ages and only have a short time together. The zero-stress option is going to be the St Martin's Courtyard for a light pre-concert meal, and leave the slap-up feast for another time when you have all evening and all of London to choose from.
This is all true but we have tickets for St Martin-in-the-fields and my dad can't walk very far or fast right now :(