Court action for unpaid legacy

I have a client who is due a pecuniary legacy from an estate. GOP issued to lay executor (one of the two residual beneficiaries) and estate solvent. Estate appears to have been administered without legal assistance and estate now distributed in full to the residuary beneficiaries. Executor claims they did not realise the legacies had not been paid - unlikely. They refuse to pay legacy. Anyone able to confirm if this would be dealt with via usual CPR as perhaps a debt claim (client is a small charity)? Client wishes to send a letter before action and would like to check the relevant protocol. Many thanks.