Tenants complaining of a smell

Other

Hi,
Our tenants have just moved in a month ago and have been complaining of a smell in the bedroom (like b.o) for the last 3 weeks.
We have been round twice to investigate and can't find the source of the smell they have been complaining about. The only smell we noticed which was going round sniffing different points of the bedroom was coming from the cupboard doors. They smelt a bit musky but not of b.o like they are describing.
The second visit we took a builder round to check the pipes, electrics and the vent in the bedroom and he confirmed it wasn't any of these and also agreed the cupboard doors smelt a bit musky.
We have offered to take down the cupboards next week to eliminate the smell but are now saying they want to end their contract early due to the smell being so bad that they can't live there no more. The lady is in early stages of pregnancy and knew this when viewing the flat but I feel that is the real reason they want to move out early as its a one bedroom flat and now perhaps having doubts about living there with a baby.
Do they have the right to move out over a smell that they can't prove is as bad as what they are saying?
Thanks
Gemma