Thanks to everybody for the advice. I have since contacted oilman who has suggested a range of 5w40 synthetics given my cars spec and usage. As soon as the old girl has completed another 250 miles, I will be sorting out the service. It's a toss up between the following all around £30 for 4/5 litres.

Motul 8100 X-Cess, Millers XF Longlife, Fuchs GT1 XTL/Supersyn, Gulf Formula G, Shell Helix and Mobil Super 3000.