I then stuck the magmount antenna onto the car roof and repeated the test. I found exactly the reverse - field strength was higher on UK40. The difference is that I cut the mobile twig to a low SWR on the UK band.
I suspect then, that the handheld twigs are set for EU. The telescopic can, of course, be shortened easily but I'd have to pry the top off the stock antenna and make tiny snips.
I also found that the field strength shot up if I gripped the handset with both hands, one at PTT level and the other on the bottom. I guess I'm a counterpoise


