Firstly, A Warm welcome to both Antony & Bartley,
Hopefully we can help you out, it is always nice to see people coming back into the Hobby.
I do remember the Guillows Kits very well, there are more around than you think, I usually see these in quite a few model shops and to be honest I have been surprised to see them still available. They are nice kits though and ones that I have indeed toyed with the idea of buying and converting for micro r/c.
Cox engines are not as scarce as you think as well. They were certainly good little engines and if looked after, they will certainly last sometime. There are plenty of Hits on these if you do a google search.
Have a look at this link as this place does both the Guillows Kits and have Cox Engines and spares too.
http://www.airfield2000.com/
The Prices are in Dollars, given the current exchange rate, you will be looking at least half that price in Sterling.
I have seen that the engines are the Cox TD .049...slightly more powerful, If this suits you you could also consider the British made PAW Engines....they also do a .049 and are superb quality engines...I have 2 of them that have lasted me for years.
You can find them here:
http://www.eifflaender.com/index.htm
Good Luck !!
Regards.......Mark.