Dom-Can't remember exactly what you need to get the seller to sign but when you have got all the relevent paperwork you will get a temporary export log book called a "Internationaler Zulasssungsschein" from the German DVLA local office. You will need to get the old Log Book called "Fahrzeugbrief" from the seller.
Once in the UK you will not have to pay duty provided a) that you're buying the vehicle from an EU country and b) Duty (ie VAT) has already been paid in that country. You can drive the vehicle in the EU on your temporary insurance and that means that you can get it MOT'd (you don't need to produce insurance to get an MOT issued-don't forget that you will need to get the headlights to dip the 'other way' and you may need to swop the fog light to the 'other side') but you will need UK insurance (and MOT Certificate-not the German "TUV")to get the vehicle registered.
Cut and paste the pdf link I posted above to get all the info on registration and if you search Customs and Excise web site you'll find all the info you need on customs duty.