In reply to Jonathan:
Thanks guys. Reading around a bit, I think my qualification is somewhere around day skipper level. I was/am hoping to start there if possible, and slowly build up my experience. I'm a pretty cautious person, and I learnt my climbing/mountaineering skills that way, gaining some base knowledge and then learning by putting it into practise, and slowly building up experience at one level before moving onto the next. I would say I'm a competent and safe mountaineer, despite having no formal qualifications. Fortunately, a lot of the boat charter places in the UK seem to take a sensible approach to these things, and most seem to suggest that if you don't have the qualification, you can call them and have a chat about whether you have enough experience. In Germany, things tend to depend a lot more on exactly what qualification you have.
I realise I will also need to do a VHF license, that was more or less my next goal, I'm hoping it won't be too tricky to do it here - it seems a substantial part of the exam is understanding VHF signals.... in English
I reckon I can manage that!
If anyone has any more particular suggestions of places to charter, I'm all ears.
Thanks!