Just read elsewhere that its a good idea to try removing stubborn plugs when the engine is warm, so I'll take it for a run tomorrow then have another go.

I'd still appreciate advice as to whether penetrating oil or similar would be OK.

Any release agent/fluid will be no problem used in small quantity.Defo get the engine warm.What tools are you using to remove the plugs?the ones you have may not be adequate.

Thanks for that. I haven't changed a set of plugs in many a year, so I had to start from scratch, tool-wise. First I purchased a 300mm, 16mm gauge spanner with an integral turning bar, only to find that as you move from left to right, vertical access to the plugs with such a long, rigid bar is made impossible by the plastic windscreen scuttle. So I'll need to purchase a hinged T bar spanner.

I'll get hold of some release fluid, warm up the engine and have another go.

You need the right tools for this job.A spark plug socket,extension bar ,and a good solid ratchet.Spark plugs can get very tight.

Thanks again to all.

I did consider removing the panel to access the plugs, but I used to have this car serviced by a mobile mechanic and I'm pretty sure he could always get to them without removing any bits of plastic first. Presumably he had a more comprehensive collection of tools.

Will tackle this over the weekend once I have the tools and the release fluid. Oh, and a plastic bag ;0).

Sounds as though a cack-handed mechanic has not bothered with a torque wrench and screwed them up like head bolts. I've always been scared stiff of stripping the aluminium head thread, so once you have loosened them while warm, do them up again, then loosen more, then do up a bit, and so on, coaxing them out.

When you have half unscrewed them, get a thin tube and blow out any debris round the edges so grit doesn't fall into the cylinders.

Also, don't change them if they look OK - I took mine out at 32,000 just after I bought the car and they looked nearly as good as the new ones I'd bought for it, so I put them back in at 85,000 and I have only just recently changed them at 102,000.