Thanks DrByte

I asked the host company to look at the details, this is the response:

"Hello,

I have just accessed the website with https://properjobpopups.co.uk on my HTC Android Phone and cannot see any problem.

If there is a problem appearing in one particular browser, that must be to do with the way that browser handling the connection.
It may get error 500 for improper handling.
Please confirm whether you are seeing problems with any other browsers on mobile devices or ordinary computers.
Not all mobile browsers are expected to work correctly by design.
You may try using Opera Mobile, which a mobile version of a major browser package."

I haven't got access to other mobiles so I can't check other browsers - any feedback from the community would be very welcome

Derek