menu not working on iphone or ipad

I have just built and released a site for a friend of mine using the t3-bs3-blank template.

I tested it on Google Chrome, Firefox, Internet Explorer 11 and my Android phone... it worked fine so I was happy to release it.

However, as soon as I did, he tells me that it doesn't work on his phone, his apple mac, or his girlfriends tablet!

So.. i borrowed my wife's iPhone 5c and surely enough - the collapsed navbar just doesn't work at all.

You can see the site here: www.gardenandponds.co.uk

I have been googling this for well over an hour and trying various fixes - but nothing seems to fix it. Is anybody able to tell me why the dropdown menu (collapsed navbar) isn't working on an iPhone?

I'm stuck

Thanks muchly

Kev