Dilemma: stop off needed at Bristol

Could any of you kind experienced CC members recommend a site close to Bristol. As most of you may know from my previous posts we are newbies and have bought a long twin axle Buccaneer. We have booked our second venture out, to Cornwall next week ( our maiden trip was last weekend Starthclyde Country Park)  travelling from Lancashire. However, my dilemma is, we now have to call at Bristol on route, as my son is having his race bike rebuilt. I need to have his current bike at post code BS3 4EG for Friday morning at 11am, pick it up at 4pm then continue on to Cornwall Friday night. My husband is new to towing ( has done the manuovering and towing courses) so I am trying to logistically fit this detour to Bristol in. I don't know Bristol so very much doubt it would be wise to try and access the shop with the caravan in tow, so am trying to rack my brain as how to unhook and leave caravan somewhere safely throughout the day? I thought of booking in at a site but they don't allow entry until later in the morning and as I said we need to get into Bristol with just the car by 11am.  Also the closest site is fully booked anyway. If anyone knows of a site just off the route to Cornwall from Lancashire, close to the Bristol area; that also is easy to access we would be very grateful. Or any suggestions of where a caravan can be unhooked and left safely for the duration of the day possibly? Many thanks!