How to remove shearded off spark plug stub

Having applied Plus Gas for a week or so, I managed to get all the visible nuts, bolts, and studs moving on my MK10 Villiers. The only casualty was the spark plug. It was seized solid and I couldn't budge it so took the whole job to the local garden equipment workshop. They successfully sheared off the top of the plug leaving me with the threaded portion still imbedded in the cylinder head (at least they didn't charge me !)
Any suggestions as to the best way to remove the remains of the plug? Thanks.

Hi Woodsman - for me nothing beats a bit of elbow grease and a good set (but not expensive) of stud and screw extractors offthe likes of fleabay or your local automotive tool shop/ engineering supplies. They are simple to use - perhaps a little bit of drilling required with a good quality metal drill bit and then tap in and then counter screw in the thread - you then appy elbow grease to a spanner or socket - the more you apply the more force involved - so ensure the extractor is bedded in VERY well. - this and perhaps a little more plus gas or heat will be a good way to start - no magic answer I am afraid....its one option - and probably the least cost option to start...

let us know you get on...

Best

Hamish
(I've used this method many times to good success...)

i just use drills going up in size 0.5mm at the last until the sparkplug remains can be folded up into the middle, or tapped around carfully with a center punch free from head threads, ive seen heads were people have knocked the cerramic center out and then used a hacksaw to cut it  into two halves ,but also then you have cut 2 es to push up the side of the new plug rendering the head scrap as compression leakage and then combustion becomes a miniture blow/cutting torch 
kev

Thanks for your comments - been applying plus gas / heat/quench for some time now to no avail. Also tried the junior hack to cuts slots on the inside but as Kev says I'm scared of ruining the head. Advice from my friendly motor mechanic is to take it to a local engineering firm that I have used in the past. Think I'll spend the money rather than make it any worse. But thanks again for you comments. Have posted new thread regarding getting at points/coil.