I found a solution without the need to use the Kaspersky forums. The problem is Kaspersky's 'Web Anti-Virus' which is the bit of the software that checks for virues sent over the internet. I've added an exclusion for the Rain Radar overlay and it's solved the problem. For anyone else using Kaspersky who gets the same problem, you need to open up the programme, click on settings, which wil open up a new layer. Then click on 'Web Anti-Virus', then click on settings for 'Web anti-virus', which will open up another layer. Click on settings, then click on the 'Trusted URLs' tab. Click on add, then enter the following *nwstatic.co.uk* (with a star either side). Doing that has fixed it for me.
Not happy with Kaspersky, it's served me well up to now, but if my 'computer literacy' had been average I might never have worked out it was at fault, let alone found a solution.
Paul, thanks for steering me towards the web anti-virus