Wilcrick services would have been on the M4 immediately west of J23a. J23a didn't exist when the services were proposed, so it's not quite as mad as it may look now.
There are three 'ghost slips', with the highway boundary (the fencing around the motorway) leaving space for a westbound offslip. The eastbound sliproads were disconnected from the motorway when the road was widened, and have since deteriorated. The eastbound onslip isn't looking to good, either.
Now Magor services are only half a mile to the east. At the time, Aust would have been the next service area, being 11 miles away. For a while it would have been the last services on the M4. Today, Cardiff Gate is 15 miles away.