Claimant filing suit in a representative capacity as executor of his mother’s estate - Death of the claimant prior to trial - Devolution of office of executor on death where there is a surviving co-executor - Inapplicability of the chain of representation - Suitability of applying for administrator ad litem by executor of deceased claimant -Applicability of CPR 19.3, 19.2(5) and 21.8