OP I understand that this is actually a really unnerving situation for you. Let's face it, no matter how amateurish the letter may be, nobody wants to receive a threatening letter from a solicitor in the post. Especially when you can't explicitly confirm the validity of it, or if there are any real grounds for litigation on the hotel owners part.
However, ANY decent (or even not decent for that matter) will tell you that defamation claims are notoriously difficult to prove. Firstly, there are a number of defences to defamation - most notably being truth. Is it down to you to prove that the statements presented are both truthful and factual based on your experiences. The burden of proof is on you to prove that. Furthermore, in the absence of truth, you can rely on the defence of honest opinion. If you can show that the statements presented were that of an honest opinion on your part, and that any reasonable (feel free to reach your own conclusion of what you find reasonable!) could have also reached the same opinion based on the facts you present, then you are protected against a claim for defamation based on that. Quite frankly, a quick scan of the tripadvisor reviews shows that the opinion formed by you, is shared by a number of others having visited the hotel.
The amount of money it will cost to being a defamation claim against you is so far beyond the realms of possibility for a man who can't even afford to fix a crumbling roof in his hotel, that any concerns you have are misplaced.