Result !
Squirted some Arctic Hayes Crack-It shock freeze release spray (from Screwfix about £7.50) down the hole. Waited a minute before inserting a tight fitting 16mm socket with extension piece and 12" bar. Gave it a tug and bingo! Out came a rusted and gunged up spark plug.
New plug inserted compete with a smear of Copperslip and torqued to 15ft lbs.
Took mental note to remove spark plugs on an annual basis. Feel a project coming on to fabricate a suitable aluminium cover plate.
Thanks for everyone's really useful advice on this.
