Right the best bet will be if you cable up from the tv with your own cable and see if you can poke it out of the soffit by the dish .can the engineer plonk his ladders directly under the dish no new pitched roofs or conservatory in the way? New dish is a must as is a new lnb . If it’s hard to get the cable out of the soffit due to head hight inside the loft push it from the outside.
It my be a cow of cable run to do in one go from the tv to the dish.. If it is do the cable from your tv to loft hatch, dish to loft hatch leave plenty of slack so he can barrel the cables without going into the loft . Not best practice but fine as long as your cable run is under 30mts.