Clive wrote:

Basically I'm taking a £100 gamble on the dehumidifier to improve the
situation for the tenant and I can't make the situation worse.

Agreed; though the problem with buying a humidifier for a tenant is that
once they realise how much they cost to run they tend not to want to
know; furthermore unless you get a model with a permanant drain they
don't work once their reservoir gets full and need emptying...

David