It's unlikely to be cheap, it sounds like valve is the only option for your symptoms as Ixion likes nothing more than to bicker among itself so if another problem were likely I think someone would have mentioned it (I'll update if someone does.)
As you know the sparkplug was trashed it sounds like it's been removed. I don't know how the angles work but it might be possible to have a look with a torch (or even just a gentle poke around with a screwdriver) to work out what state the piston is in as that would give you a better idea of what has happened.
There was a suggestion that engines are going on eBay for around £250 and I guess it's half a days work to swap one out (that is a seriously finger in the air guess mind) a phone call to any of the mechanics you've had suggested and a quick chat should get you some more concrete information to work from.
no subject
As you know the sparkplug was trashed it sounds like it's been removed. I don't know how the angles work but it might be possible to have a look with a torch (or even just a gentle poke around with a screwdriver) to work out what state the piston is in as that would give you a better idea of what has happened.
There was a suggestion that engines are going on eBay for around £250 and I guess it's half a days work to swap one out (that is a seriously finger in the air guess mind) a phone call to any of the mechanics you've had suggested and a quick chat should get you some more concrete information to work from.