In reply to FesteringSore:
> I've managed to identify the base unit - KX-TG8061E (Panasonic) Should it be a simple task of purchasing a couple of additional handset?
Do searches on eBay for KX-TGA 806 (extra handset) and KX-TG8061, KX-TG8062, KX-TG8063, the last digit in the model number refers to the number of handsets, so for example KX-TGA 8063 comes with three handsets, one base station and two charging stations. If you were to buy a KX-TGA 8062 set, currently on eBay for about £25, you would end up (including the set you already own) with one charging station and two base stations but the instruction manual covers how to code all the handsets to one station and then just use the spare base station as a charger only.
In my experience different Panasonic models work quite well with each other so you needn't be limited to the exact model you already have, the KX-TGA 8061 base unit only has 18 minutes of recording space, fine most of the time but if you're away on holiday for a few weeks that can easily fill up. If buying a different model look for one with colour screens and 40 minutes or more recording time, if buying second hand then you'll probably want new batteries, any AAA NiMh rechargeables will do.