Section 20 Query - No Notice Served, Demand For Over £20k

Hello all,

Hoping for some advice here. My elderly and vulnerable father had issues with his previous freeholder trying to forfeit his lease. This was thankfully resolved and the freeholder subsequently sold their maisonette and the freehold.

He lives in the upper maisonette, the freeholder owns the lower maisonette and is renovating it.

The roof needs replacing. Prior to the sale, it had been agreed to replace the roof, quotes had been obtained and a figure of £28,000 was agreed. This was without a Section 20 notice.

The new freeholder initially said that he didn't propose charging my father anymore money for the roof works, however this has now changed.

The freeholder is experienced in property and has done multiple developments, always using the same firm of builders.

The estimate for the roof replacement now stands at £70,000 and he is demanding payment in full of my father's share (so an additional £21,000) within 28 days. No Section 20 notice has been served. The builders who are renovating his maisonette have provided a quote to back up this increased cost, but we don't have any comparison quotes, and these builders have a very cosy relationship with the freeholder.

Am I correct in thinking they have not followed the correct procedure in this? What are my options? My father absolutely wants the roof replacing, but equally doesn't want to pay an excessive sum.