Plumbers & gasmen & electricians - they know we don't want to fiddle with these things, for fear of flooding, unexpected combustion or electrocution. Dence they figure they can charge what they like. And so they do...
Harpmaker - if your leak is somewhere accessible, try wrapping self-amalgamating tape (which you should find at your local hardware stoire, B&Q, etc) tightly round it till you can find someone reasonable. If your leak is visibly at a fitting, it's even worth just trying to do it up a bit - they can come loose.
jOhn - I suggest your local greengrocer... onions ought to be about 30p/lb!