Wilcrick services would have been on the M4 immediately west of J23a. It was the only service area in Wales ever planned by the government, which suggests it was proposed by the Ministry of Transport in England rather than by Wales's local motorway engineers.
Most references to Wilcrick services we have found come alongside other services on the M4 in England, which adds to that theory. The idea would have been that Wilcrick was reserved for when the previous service area, Aust, became too busy.
When the Welsh Office took full responsibility for Wales's motorways, they revealed that they wanted as little involvement in service areas as possible. This meant it was much easier for interested operators to buy some land next to a roundabout - which is what happened one mile away at Magor - than for them to try to negotiate with the Welsh Office about building a new exit on the M4.
Incidentally, Aust services never really reached its full capacity before the M4 was diverted and it virtually closed. Magor was built to serve the new Severn Bridge, not to relieve Aust.
The planned site has 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 too good either.
The construction of M4 J23a means that no service area could ever be built at Wilcrick now anyway, as the two exits would be too close.