Having personally employed plumbers, here are the details:
Plumber with just 10 years experience in London on the payroll will earn between £12.50 - £17.00 per hour with a company van and all fuel paid for along with travel time.
Plumber with ACOPS (Old corgi) - £15 - £20 depending on direct experience along with the perks above.
Sub contractor - normally £17.50 per hour straight - 18%.
Its not that there is a shortage of plumbers, its the fact that there all incapable twits. Heres something that someone told me once and its never a truer word said:
All a plumber has to do is look presentable, expalin a decent report and be polite and he'll go a long way.
The fact is, none of them appear to be able to do this.
I would say Carpentery is the best trade to get into. Never out of work, and earn quite a bit of money (in some instance more than plumbers). They are always presentable and its a more skillful profession IMO. Being a carpenter, other skills will come to you such as locks, glazing etc. With a plumber, there is nowhere else to go and you really do stink.
Roofing is OK - but generally out of work in the summer months unless a big contract is drawn for a roof overhaul. Earn good money, but can be quite dangerous.
Electricians - Good again. Presentable and can get you into other areas such as Networking.
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Sparkies earn around £15ph on payroll with company car and paid holiday etc.
They normally get the easiest jobs out of all tradesman as believe it or not, they actually get called out to replace light bulbs, revise timers, and do electrical reports. Its pretty simple. Its the theory exams that are slightly difficult and its certainly the most technical and professional of all trades.