Please search for "Roundhill Barn Kelston" (Google will confirm "venue for celebration"). It's not a registered address, and the nearest post code (BA1 9AF) will not take you to the correct location. To further confuse matters there is another "Roundhill Barn" in the village of Kelston. Sorry about that.
Best options are:
From Bath on the A431: just after you see “Bath Soft Cheese” sign and 30mph limit turn right off the A431 on the outskirts of the village of Kelston on the second of two farm tracks (sign reads “Old Barn” and “Brocks Outdoor Adventures”)
From Bristol: turn left off A431 just as you leave Kelston, on the first of two farm tracks (sign reads “Old Barn” and “Brocks Outdoor Adventures”)
Please avoid going up Cullimore’s Lane - there’s no vehicle access and no turning.
Enter through the gate at the end, turn left and follow the track up the hill for half a mile.
Google Maps street view image of the turning off A431: https://goo.gl/maps/uobup3NzFWrWSEcC9
There is space for 50 cars - drive up the track to the left and park in the second field, to the right of the track. It is possible to drive right up to the barn to drop off mobility impaired guests or deliveries.
Please advise taxi drivers to search for The Old Barn at Kelston. (Warn them not to go to the residential address Roundhill Barn, Kelston BA1 9AF)
Public transport: Kelston is served by the #19 and #37 buses between Bath and Bristol. Get off at The Crown (have a swift half; why not?) and it’s 12 minutes walk via Cullimore’s Lane.