Another sad S8 owner because of the gearbox

[completelylostagain]

So the S8 flew through it's MOT after two years off the road, but I have the classic gearbox problem.

I took the car for a drive last night and there was a little slippage, but today after the MOT big slippage, bangs and PRNDS.

Turning the car off and on resets the PRNDS, but the slippage and bangs are back.

The car, at first, doesn't seem to engage and then I get the bang, and Reverse works just fine and doesn't set off the bangs.

My mechanic who seems very competant and is an Audi specialist (and who is now on holiday for 3 weeks, spending all the money I have given him) changed the oil, but didn't leave the transmission to drain, and could only warm the gearbox using idle: I know that's not the full reccomended process.

I think my options are:

1. Have the transmission drained overnight and refilled as per audipages by an audi specialistgarage to see if that fixes the problem
2. Assume the gearbox is terminal and get it serviced by a ZF specialist: but how much?
3. As 2. above and get it serviced by the guy who does it free, but where do I find his details?
4. As 2. but get a replacement from A8 parts, take the risk that its secondhand and get the original refurbed if needed at a later date either paid or free.

At the moment I'm thinking of 1 then 4: any thoughts?

If I drive the car in limp home mode will that cause any problems>