Hi Centralheatking,
I agree, Cornwall is a lovely place and I to would love to retire there as well! Just a thought if you are looking to try and find work, what you could do is when you are next in Cornwall a "small marketing exercise" to try and gauge what the demand for plumbers is in the area you are thinking about semi retiring to.
Just a few thoughts of the top of my head. Try a very direct approach...knock a few doors of local people, introduce yourself and explain that you are thinking of moving into the area and setting up a plumbing business and would they be interested in using your services and just see what reaction you get from the locals.
Another suggestion is put your business card in some local newsagents and see if you get any calls from potential customers.
You could also run a mini leaflet campaign in the area you are hoping to move into. (ÂŁ37.00 for 5,000 leaflets available from cheapprinthouse.co.uk)
Maybe telephone a few builders and see if they need a plumber. Again they will give you an idea of the demand for plumbers in the area.
Maybe run a few adverts on Google Adwords for Cornwall and see how many clicks and telephone calls you get from that.
Once you have done some or all the above methods, at least you will have an idea of demand for your services, and it will give you a better idea if there is enough work available. As you probably know once you start talking to people it can throw up all sorts of opportunities. For example, you may find that as there are so many retired people in the area, they would really love a plumber that provides a regular servicing contract as they need their heating to be reliable and they want peace of mind. For the cost of about ÂŁ100.00 I think you could very quickly find out if it is feasible to set up in Cornwall.
Hope the above is of some help
Simon