We have recently started using more frequently the shower over our bath. We have only ever used the bath before as we have an ensuite shower. Not long after using it we noticed a smell of stale water. Not sure where it is coming from. Don't think it is the drains as we have had baths before with no smell. We are concerned it may be leaking but can't see anything and not sure if a leak would result in this type of smell so quickly. We have tried to get a plumber out but can't get one until the new year. Any advice would be appreciated. Thanks

Your nose will tell you more then I can. Where's the smell the worst? Look down the drain at all the traps. If they're full of water then you should be OK, but if there's almost no water in any of the traps then you could have a vent that's blocked. Also you've tried everything else you might want to consider replacing the wax rings in your toilets. Sewer gas can escape a defective ring yet not leak water when flushed. The wax ring can emit sewer gases from the base of your toilets and you will swear it's coming from the shower/tub drain, lavatory drain etc.
Sewer gases have a way of dulling the olfactory nerve and make detection nasally impossible.
You must realize that smells and weird noises are the hardest to track down when you're on site and just about impossible when you're not.
All I can do is point out likely spots. Good luck, Tom