Hi folks,
Sorry for the long, long post, but who knows what detail might be important to the trained eye

After many happy miles (from 64k-150k) with a 1987 190E 2.0, I bought a 'bargain' 1989 190E 2.6, 109k, full service history, one elderly user, who serviced it every 3,000 etc.
For a couple of weeks, 1,000 miles, it was a flying machine (though I didn't thrash it) . I had a full service (plugs, oils, filters, fanbelt, plus usual checks) - still fine, then...

Maybe coincindence, but after unusually driving in city traffic for a couple of hours (no visible signs of overheating), the idle gets rougher, though it's still easy to start. Engine seems to lack a little performance as if the handbrake binding!, or low-octane fuel. No weird noises, no massive oil consumption, a little bit of parping from the exhaust - though it's always sounded a bit more boy-racer than my old 190.
I'm commuting up the motorway a week or two later, when power seems noticably down (still able to do 70mph). I pull over into service station, listen to engine (nothing odd, though it's shaking around a bit!), gun the throttle - no smoke, a tiny, tiny backfire.
Back on the road, now it struggles to do 40mph, off the road again and get breakdown recovery.
He checks spark on all six cylinders - all fine. Then pulls lead while engine running - three cylinders down (3,4,5 counting from front of engine) so that the idle doesn't change when you pull off lead. Four and five look a bit wet (not oil?). Diagnoses blown head gasket. BUT no overheating, no loss of coolant, oil looks fine. Flatbed truck home.

My regular mechanic, who originally guessed split rubber somewhere on the inlet side, checks compression - "none" on 3, others ok. 4 and 5 plugs (300 miles old at most) covered in crud, ?carbon?. Now he guesses, in order of horror: broken rocker arm, burnt valve, holed piston, and I'm trying to be a big boy and not cry.
He is going to lift the top cover and check out his rocker arm theory, then it would have to be head off etc...

My questions are:
could it be something much cheaper, please? is my mechanic ignoring the obvious somewhere? might injector replacement/clean clear this whole mess up for me, without surgery? I've read the relavent posts but can't work out if this might be a cure at this stage, or just a cause.

I paid £1100 for the car ($1900?) - I love it and would pay the £500-ish for the top end job, replace the valve(s), guide seals etc if I knew we'd got to the cause of the problem too...but I'd need to cut my losses now if the car's going to be a money-pit - I've got a 110m/day commute.
Lucky I hung on to ol' Rusty.

Thanks for any advice you can offer,
best wishes
Alex
1987 190E 2.0 150k
1989 190E 2.6 110k