I took out the bottom bracket to clean it etc taking off the pedal shafts etc - and when it came to putting it back - the front derailleur was well out of adjustment - Have I done something wrong>- This is the first time I have taken out a bottom bracket. Thanks Ben
I'm assuming this is a 3 piece Shimano sealed-cartridge square-taper style bottom bracket, since you didn't provide any further info, and that fits the price/style/vintage of that bike. (for more on bottom bracket styles, read the always useful Sheldon Brown site). Additionally, how is the derailleur out of adjustment? Is it a cable tension issue? Is it not a front derailleur position (yaw, height) issue from getting moved during the operation?
Assuming you did not alter the position of the front derailleur, and following my assumption of the BB type, you probably did not install your bottom bracket properly as @Hicks was getting at. Most 3 piece bottom brackets have 2 cups with the threading, and the bearing/spindle set itself which fits inside the cups. (If not, the drive-side threading is probably fixed to the bearing/spindle assembly (but also possibly not)). These bottom brackets are generally larger than the bottom bracket shell of the frame (depending on manufacturer), which means the bottom bracket's threading will be protruding from at least one side of the bike. If the drive-side of the crank spindle protrudes further from the bottom bracket shell than it should, your front derailleur will always be off by that distance.
When installing the bottom bracket, you must fully thread the drive side first so that it matches flush with the bottom bracket shell of the frame. Then you can thread the non-drive side (for further instructions, see Park Tools Guide. Re-attach crank-arms, recombombulate bicycle, etc.
After following this process, your front shifting may still be off--the factory or mechanic that built your bike may have adjusted the front derailleur with the bottom bracket not fully-threaded in on the drive side. If it's still not shifting correctly I'd suggest learning to adjust your front derailleur, because that's a much more worthwhile endeavor than trying to improperly install the bottom-bracket exactly as it was when you got it/had it repaired last.