Entourage to I cal

I have an I mac using 10.4.11 and use Entourage as both calendar and email client. I am located in the UK, and have my clock and time zone set accordingly, however, when sending a meeting request to my wife (Mac book 10.5, ical) the time is always 1 hr off (if i request 10.30 her machine will show 11.30) it is only a problem with my machine, and does not do the same thing when a meeting is requested from outlook on a different machine. I have checked the obvious such as time settings, and it only does it when we are on BST, which i am sure is significant.....
Many thanks,
N