2002 A4 TDi 130 vibrating

Hi

I have an issue with my A4. It's 2002, owned by me from new and there's now around 80,000 miles on it. Since 2006, my wife has taken it over and it does less than 2,000 miles a year. It has never failed its NCT and it's serviced annually. At this stage it's had the timing belt changed a couple of times, most recently last year.
The only ongoing issue is it goes through batteries as it isn't driven enough. Last week I had another battery put in and drove home normally. It was the following day when I noticed an awful noise at idle. I don't think it's in any way related to the battery although it initially sounded like something was hitting off the engine and vibrating.
I pulled the bonnet and immediately noticed the engine was vibrating more than it should. I know it's not the quietest engine but it was like a bag of hammers.
Once warmed up, things improve but you can still hear a background rumbling.
I thought about a loose exhaust or even a clutch issue as you can feel something when driving and changing gear.
This morning, I brought it down to a local garage and after driving it, the feeling was a worn camshaft, which isn't unknown with these engines.
When we took the oil cap off, there's blowback that you can see.
The car has always had good oil and the level never dropped between services but I suppose it's not impossible.
It's going back on Thursday for the rocker cover to come off so we can have a better look.
How likely is it that the diagnosis is right and if it does need a new camshaft, what kind of cost am I looking at?

TIA