Did I issue an invalid s21 notice?

I lived in my property for a number of years before deciding to rent it out. I had my friend who was previously a registered gas engineer fit in my new boiler and service it for me (while I was still living there)

Just before my tenant moved in 2018 I got him to do another check so my tenant could move in ( he was no longer gas safe registered on these 2 occasions).

I issued my tenant a s21 accelerated possession order which he defended claiming he knows the gas safe certificate upon his tenancy is not valid due to the name and gas reg number not being on gas safe register at this time so this makes the s21 invalid.

I have since received a court order stating ‘judge is not satisfied claimant is entitled to use accelerated procedure’ and to attend a 10 minute hearing in 4 weeks time. I am in slight pickle because I didn’t have any further checks done for 2.5 years because I thought it had to be done every 5 years, and when I did have this check done it came back at risk to the flue not being sealed.

I just want my house back to sell now. Has anyone been in a similar situation or could offer me advice?

Thank you

Danny