We are trying to fix a leak in our bathroom, we thought it was the tiles, but fixed those and then realised that water has got under the lino when we pulled it back the floor underneath is wet. Underneath the floor is not floor boards, but a sort of big board, maybe floor boards under that. Not sure how to fix the problem. Should we take up the big board? If we want someone to fix who should we get, surely this isn't a plumber problem.

The floor is wet from water pooling there after a shower. I don't think it is a leaky pipe. We recently had electricians in who lifted the lino and it is since this that the problem has arisen.

A wet floor in a bath is a plumbing issue and must be fixed before you think about replacing any flooring. How did it get wet? Most common reason is a dripping drain or supply line followed by a bad wax ring under the toilet. At this point everything sounds soaked completely so a complete removal of the flooring may help expose the source of the water

By "lino you do mean the tub liner don't you?
. We recently had electricians in who lifted the lino and it is since this that the problem has arisen. I'm curious, What in the world were the electricians doing in your bath tub and why did they lift the tub liner?? That blows my mind. You are correct that the problem started after they left because they caused the leak. Let explain. As you can see by the image I've put up the upper part of the drain has a lip. When we install a tub we always make a small ring of putty and place it under the lip of the upper drain lip before tightening it down to form a seal. When a liner's installed we unscrew 5the upper drain and install the liner leaving the drain for last when we make a new putty seal and then screw the upper drain back down on top od the liner to reseal the drain. When your electricians lifted the liner, ( still trying to figure that one out) they ruptured the drain seal so now when you shower the part of the water goes down the drain and the rest on your floor.
The solution is simple. Simply unscrew the drain and replace the old seal.
Good luck and happy showering. Tom PS. In my world I would first complain to the electrical company so they know what's going on and request them to repair the tub and any water damage that you may have on their dime with no cost to you. T