Hi I live in a 1950s house and have wooden floorboards downstairs I lifted some of the boards and found damp mud is this normal as having trouble keeping the house warm. I have underlay and an axminster carpet and you have to sit with your feet up or they freeze why is this?

Not normal. Is there concrete under the dirt? There needs to be some water (vapor) barrier under the wood; so the wood doesn't get moist and mold and rot ensues. There may be a big crack in your foundation or floor allowing water to take soil into your house. You need to clean-up the wet mud, repair your cracks, put down a proper concrete floor and then replace your wood. A wood floor over dirt hasn't been allowed since the log cabin days. Hope this was helpful.

If you have dirt under the wood, you have drainage or leakage issues. If this continues you will have a lot of damage to repair. Is this on a slab or is this a basement?

My house was built in 1940, with many add-on rooms since then. The old flooring is on 2 x 6 boards, laid on concrete foundations all around the outside of the house. The flooring is open to the ground, on top of the 2 x 6's.
As far as I know, the ground underneath is dry. I can look through a window in the basement, into the bottom of part of the house, and it's dry.
I would talk with a contractor, and see about putting a barrier of some kind around the outside bottom of the house, to keep water from draining in from the front, sides, or back. Good luck. I am not a carpenter, contractor, or professional builder. This is just my own opinion of where I live.

All crawl spaces should have a vapor barrier between dirt and living area. Something as simple as a sheet of plastic held down with gravel will do. All unheated crawl spaces should also be vented. Your local inspector/ building inspection department is there to give you the proper specs recognized in your area.