In reply to Stuart S:
It can't happen very regularly, but we took a small rack of stuff up to Glasgow from Heathrow over Easter. It got completely and utterly mawled by BMI. (After a bit of arguing they agreed to pay replacement costs.) We are talking about a 60m rope into about 20 peices, harnesses (each) into about 10 peices, half a shoe left out of two pairs and crabs broken. In a rucksack which was in a ruck sack "protector". (We couldn't work out at the end whether or not the outer protector bag was still there, the bags were in so many shreds.)
The problem is that by the Warsaw convention airlines are limited to a payout on hold-luggage of £13 per kilo. If they argue about paying up, you have to Prove that they deliberately or recklessly caused the damage (which is obviously pretty difficult after you have given it to a lady at the check in and the next time you see it is 1000 miles away). You could, of course, go on to your insurance, but then there are all kinds of other difficulties.
I will think very hard about ever taking expensive stuff in the hold again. I would do my best to investigate what they will allow on the plane and take it that way if possible.
Definately Do Not take your helmet in the hold, take it on the plane.