Just wondering if anyone can let me know where I stand with a local garage who managed to shear off a glowplug during a change?

My 2008 Vauxhall Corsa 1.3CDTi (~39K on clock) showed a warning light the other week - the one that's the orange car and spanner - so took it in to get a diagnosis. The fault code they reported was something along the lines of the ECU wasn't getting a signal / talking to a glowplug. Their course of action was to replace the glowplugs.

Within half an hour, they'd rung to say that although 3 of them had come out fine, one hadn't but had in fact sheared off, thereby leaving part of it situ. They'd take to a local engineering firm for an evaluation and get back to me.

The next day they phoned to say that the local engineering firm said they couldn't remove it and the only option that I had, therefore, would be to take the cylinder head off etc. and get to it that way. This would, of course, cost a lot of money!

They said: "These things can happen, but it's rare," and that was it. Pointed out that I'd brought in a car with a warning light on, left it in their trust to fix, and now had a car that could require major work in order to rectify something that had happened when in their care - but their attitude, again, was it wasn't anything they could've been prevented.

Looking at the what's left of the offending glowplug that's stuck, the thread has sheared off at the top, leaving what can only be described as the outer body of the thread in place, with the bit attached to the connecting wire having the centre electrode (sorry for vague explanation: I'm not mechanical!).

1. Are the garage correct? Is it 'unfortunate' that this has happened or do I have any claim against them?
2. Does it really need the cylinder head etc. off? Internet guides seem to suggest not, especially when there's a cylindrical bit still in the engine that appears to be suitable for removal by easyout or whatever.
3. Would the engineering firm have said it wasn't possible as they realised that the garage has bodged it and weren't going to right their wrongs, so to speak?!