@Saywhetw The law has changed since I worked as a solicitor doing possession proceedings but I’ve done some legal research this morning.
In the non-binding county court case of Carson Property Ltd v Monty Shooktz [2018] it was held that the GSC must be provided to the tenant at the start of the tenancy and the LL is unlikely to be able to serve a s21 notice where it is served late.
However, in the case of Trecarrell House [2020] the Court of Appeal held that a LL who fails to provide the tenant with a copy of the gas safety certificate before the start of the tenancy can remedy that default by providing the copy before serving a s21 notice, so long as the certificate was in force at commencement of the tenancy.
The position may be different if the LL has not obtained a certificate at all before the tenant took up occupation or if he has not retained a copy of the certificate that he can provide later on.
In the non-binding county court case of Byrne v Harwood [2022] the judge set aside a possession order, finding that if a LL has not obtained a GSC at all before the tenant takes up occupation he will be forever prevented from serving a s21 notice. However in another county court case Cassell and Cassell v Sidhu and Sidhu [2025] the court held on appeal that the LL’s failure to provide the tenant with a compliant GSC (the certificate omitted the LL’s name and address) was remedied by the provision of two subsequent complaint annual certificates during the course of the tenancy and the s21 notice wa therefore valid. This case is also non-binding and does not align with Byrne so the position remains unclear.
I imagine your solicitor will say that your options are to either (1) negotiate with your tenant (possibly by offering her compensation) or (2) to rectify the default (including deposit protection, prescribed information, EPC, etc), serve the s21 notice, issue possession proceedings, and wait to see whether the court makes a possession order. The first option is probably cheaper, quicker and less uncertain.