I think the spindle and seals are getting a limescale buildup. It is possible to dismantle the valve, descale it, polish up all the working parts with a green nylon scourer, and reassemble, preferably with new rubber seals and silicone grease, and it should then turn smoothly and not drip or leak.
The lime deposits will probably also dissolve away if you use it in a soft water environment for a while.
If you haven't done it before, I would hesitate to start unless you can do without the shower until you have finished. I doubt you will do it in less than 2 days, and something might go wrong. You might find parts are seized together and you need to soak it in descaler overnight, or you might have to buy an extra tool or service kit.
Have a look at the price of second-hand Mira showers on ebay from people who are refitting their bathrooms. If you are really lucky you will find a mixer in perfect condition. They will quite likely need the same work, but at least you can repair one while the other is still in service. This is especially worthwhile if you have two or more showers of the same type in your home, then you can refurb and swap them in turn.
With Aqualisa, you can buy a complete replacement cartridge and just exchange the internals if you wish. I don't know if this is possible with your Mira.