If this has happpened in the last couple of days, and the plant is in full sun, the heat could have caused this, but if its only been in since this spring, maybe lack of water - they like damp soil conditions.

Bamboo it's been there a while but I have been very regular in watering it extensively? Every other day a 10 litres? Is that enough?

Sounds like plenty - is it in full sun?

Yes but only part of the day. From about 2 pm onwards. I think its wind damage myself as it has been quite windy here.

Well that's a case of Murphy's law - this plant likes shelter from very hot sun early to late afternoon, and that seems to be right when it gets the sun. Not a problem up till now because its been cool, but now, when its been very hot, that might be why this has occurred - wind might contribute a bit, but I'd expect it to look worse than this, with more extensive damage, if excessive wind was the cause - its nearly always windy in the UK now.