Thanks Cloddopper - your pearly words much appreciated.
Gonna have to take the head off though - gasket definitely blown. Obviously no idea how long it's been "blowing" and just hoping block and head surfaces are not etched/eroded/wormed when taken apart. If head warped, seems I can still buy a replacement. All fingers crossed.
Before you take the head fully off slacken the head bolts two full turns and gently tap the head with a block of wood & hammer to loosen it .
gently lift things apart the gasket may well break up otherwise.
If the gasket you can buy is not the right one & you cannot locate one try eBay 201124660328 , that gasket material is great to make them from & very useful to have a sheet of .
The company will also apparently make you a gasket for a price too .
Loosening the head bolts
Soak with WD 40 or a known good quality easing oil three time in 24 hrs .
Next day start the machine get it hot or if it won't start poura full kettle of boiling water over the head & bolts , then whilst it is still piping hot using s brass drift or aluminium bar & hammer give the bolt heads a smart clout with a 1 & 1/2 pound hammer using the bar on the bolt head to transmit the shock /impact , making sure you don't slip to the side & break any cooling fins etc.
Let it cool then gently try and undo the head bolts with a socket spanner extension & Tee bar or an off set ring spanner of the correct size held in both hands to keep the socket or spanner in best alignment to the bolt head .
If they won't budge put a two finger gentle pressure pull on th spanner or slide the socket bar through to one end take off the extension & using a 1/2 pound hammer give the bar or spanner a sharp crack about two inches back from the bolt head whilst it's still under tension & in a firm grip .
That usually frees of a lot of stubborn head bolts .
If it still won't undo try a blow lamp to do the heating but do be careful not to over heat & melt any low heat metals .
Re pitting of the faces at the cylinder head
light ones can be wet & dry removed using a fine wet & dry paper wet with alight oil on a small sheet of plate glass as a flat surface or a 1/4 inch bit of flat float glass glued to some thick plywood .
I do believe that on eBay there are also high temperature gasket sealing copper spray compounds for sealing imperfections at cylinder heads & other high temperature joints .
Here see what you think . ( I'd use it ) eBay 351782734014