ORIGINAL: VanhireBoys
Rule out the battery first Paul. Theres a test to see if the battery itself will hold the charge. If teh battery is in good nick then one of these is a great job for finding issues like this one..! If you can lend one even better !
http://www.uktools.com/auto-current-tester-maxi-fuse-p-54156.html
Bonnet open - Key out - Doors closed - Fusebox open
Start at fuse 1 and move along fuse by fuse until you find the circuit that has the current drain. Lets just say its circuit 11 thats the interior light, cigar lighter and clock.. Scout around them to see if sommits left on.. boot light is a prime suspect.. Circuit 12 points to the alarm system.. 36 or 39 means the radio ... Just keep hunting like this.
Be methodical ..Gather the evidence Never assume .. As said before if you assume you make an ass of u and me....!