A locator collar is going to cost between £150-£350 depending on what youre after and can afford, the old deben boxes are a good place to start if on a tight budget and can be found on here in the ads or ebay etc or the newer long range deben is ok dont go for the mark 2 or 3 though they were known for trouble, then there is the ortovox stuff which i use and cant fault it (D3) or the bellman and flint which is expensive but also well worth it and i cant fault that.
any of the l;ocator systems are a good thing as you can get a mark and keep digging until there instead of digging blindly at t
