156 1.8 rough running problems

Hi folks,
My 2000 1.8 156 had been sitting for 6 months with no driving (smashed up
leg so couldnt drive).
I've been driving it again in the last week and noticed what sounded like a
small blowing noise from under the top cover (silver plastic thing),
I put it down to a loose gasket and was about to get it changed.
Today all hell broke loose and I lost almost complete power but was able to
limp it home under 3000 revs.
It feels like a really bad misfire with the occasional after explosion
seemingly under the car (exhaust) but it also sounds as if its got no
exhaust attached either.
Really bad rough running (bad engine vibration at idle) and incredibly loud
as well.

I know this is a bit sketchy but does that ring a bell with anyone? Im still
disabled at the minute so cant get hoking into the engine bay to have a look
myself.
I'm really looking for suggestions before running the risk of trying to limp
it down to my main dealers.
All the best.
Joe

OK now know what the problem is.

Looks like the 2nd smaller spark plug on one of the cylinders wasnt
tightened properly at the last service (<3000 miles ago).
Its worked loose and the compression blowing back up the tube has broken off
the plug cap and melted the plastic around the top of the plug cap.
Looks like I need the gunk cleaned out, spark plug tightened and a new plug
cap.

Whats the odds the dealer will accept responsibility?
Joe
http://www.joefoxphoto.co.uk