I think you missed one detail in my message above and that is my PC is already set to an MTU of 1300 bytes which is why the ping fails until I go below a packet size of 1272. The router is set to a higher level but a value of 1460 I was told (a while ago) should work best for the GRE tunnel. As the PC has a smaller MTU than the router and it is quite small I do not think any changes to it will have much effect.
I also pointed out that 95% or more of websites work quite well with no delays or timeouts. It is only a few and now notably
www.virginmedia.com which I cannot connect to. It would seem strange if the MTU setting only affected some websites?