You could always check out a Stanley FatMax tape measure. Big hook on the end and the tape will stand out to 4m which is more than enough. £20ish on Amazon etc.
There will be others similar to it, probably for less money, but I happened to know of that one.
the nodding donkey:
Evil8Beezle:
I followed what I assumed was a high container through that bridge on the A36 the other week.
I was surprised he was taking that route, so I hung back as he went under, just in case 
He cleared it with what looked like a fraction to spare, rather him than me! 
I I’ve been under that bridge plenty of times, with the sign lit up “overheight vehicle. Turn around” . There’s plenty of room in the middle of the arch. …
I’ve lit it up with some slightly higher than average fridge trailers we used to have but kept going and didn’t hear any crunching noises!!
Thanks again everyone.
I had a large container so went the safe route.
As for buying myself a tape measure and I must be thick because I have not. I’m sorry but I have not had a chance to get anywhere that sells them on my travels.
I’m off this Saturday so ill get one then.
Thanks again Thomas