When a cable carries current it lets off heat. The more current the more heat [¹]. If it's exposed to the air it can get rid of the heat easily so won't get so hot for a given current. If it's buried in insulation it can't get rid of the heat so well and will get warmer for that same current, or to keep the maximum temperature the same it can carry less current. There are tables showing how much current different sized cables can carry in different circumstances (e.g., clipped to the outside of a wall, clipped to the top of a ceiling under insulation, buried in insulation, …). Your electrician will deal with this but it's worth bearing in mind if you're going to have any high-power tools in your workshop.
[¹] Proportional to the square of the current, so, e.g., 32 amps results in 4 times as much heat per metre of cable as 16 amps so, for any given level of insulation, 4 times the temperature rise.
Erm that sounds the wrong way around. A Wayleave is needed upfront to establish the rights of a supplier to route across anothers private land. Signing a wayleave now would reduce the rights of the OP to complain about the problem now encountered due to the uncharted incursion in his plot.