WordPress return incorrect time

I have two sites running on the same server. Both have the time zone configured to London. As we are running on BST the current time is GMT + 1. One site returns the correct value for GMT and thus the correct value for local time as GMT + 1. Th eother site returns an incorrect value for GMT. It actually returns GMT+1 in General settings instead of GMT and it reurns GMT+2 for local time.

I have a small php file on each site which obtains the time correctly. Is it possible that a theme is the case as the sites use different themes or is there another possibility. Any hints are appreciated as this has been doing my head in today.

I can set the timezone to UTC or UTC+1 as appropriate but I lose the DST function.