I'm in the process of serving notice to a tenant and in a similar position i.e 224 days, I've decided to contract a solicitor to make sure I don't mess it up. He's charging 270eur for letter, witness, registered post etc and they recommend 230 days notice in this case just to allow for any delays in postage.
I contacted the Housing Agency and they don't purchase properties that have HAP tenants that are being sold, need to contact Local Authorities. Which I will do once notice is served next week.
Luckily my lease agreement doesn't restrict the sale.