I sympathise Myriad. Replacement involves getting onto the backnut that holds the tap onto the bath. Can't be done from above. A decent plumber would get onto the backnut with a tap spanner of some sort.......... even if he can't actually get his hand right up to the tap ............. if you know what I mean :o)
Sounds like you have an old shower/mixer, so it's not just the tap washers but whatever else is inside ........... "O" rings maybe.
Nothing else for it. Decent plumber needed. A good one would soon have the job done.