"Remember the days when Laker, Virgin, carried a wing mounted 5th spare engine incase of repairs down the route".
So are you suggesting that Laker routinely carried a spare 5th engine in case of "problems down the route"? In that case, I would like to know what you are on for I think your memory is somewhat defective.
The only four-engine aeroplanes that Laker ever operated were a couple of Britannia 102s, two Boeing 707-138s (ex-QANTAS and Eagle)and two Boeing 707-351s (ex-Northwest Orient and Cathay). The Britannia was not capable of carrying an extra engine except inside and I do not ever recall seeing one of the 707s ever carrying a spare engine.
Mind you, I was only a DC-10 captain with Laker and we would have been sorely pushed to have carried a 5th engine!