There's a list of the autumn ones here: http://www.gloucestershire.gov.uk/CHttp ... =53187&p=0
Afraid I don't know how many of the schools also do them in the summer. Sorry.
Edit: Just found this list (scroll down a bit to find it): http://www.elevenplusexams.co.uk/school ... re-11-plus
It lists open days/mornings/evenings for 2013. They all seem to have had something in the first half of this year so will probably all have something in the first half of next year too, but some only seem to have mornings in the summer and evenings in the autumn, which might make a difference as to whether you were able to take your DC with you.