Prior to my recent tour of Corsica I took my bike to my local bike shop for a service. I had had strange creaking sounds coming from the vicinity of the chainrings/bottom bracket. When I went to pick up the bike they said they had changed the back cassette and chain and tightened the chainrings. They also said that they had changed the bottom bracket bearing but had had trouble finding the right type. They ended up taking the cups of the old bearing and using them to fit the new bearing. During my trip, I straight away noticed a creakiing/cracking sound when the bike was in the middle or largest chainring and the smallest two or three back gears. I also noticed that trying to climb the Corsican hills/mountains had become ten times harder than similar hills last year (that could of course be down to me).
Anyone any suggestions? I ride a Dawes Super Galaxy that I bought sometime at the beginning of the nineties. It has an LX rear derailleur and the chainrings are Shimano SG-X.

They either installed the wrong bearings and/or over tightened the bearing cups. Either way it is accelerating the wear of the bottom bracket assembly. You need to go to a different shop, better yet perform the service yourself, it isn't that difficult and the tools will pay for themselves in two services. I would suggest you Email Dawes and find out what the orginal bottom bracket specifications are for your frame model and chainwheel set. Then order the part or it's equivlent yourself
Here are the instructions for replacing and adjusting cup and spindle bottom brackets