Well, as promised, here are a couple of pics of the thrown-up dish.
It can be clearly seen that skew is miles off, which ain't exactly going to help with weak birds and the cabling needs tidying up.
When I fitted the IBU, I was standing on the boiler house roof between the dish face and the LNB clamp, and it looked to be bang on zero deg. skew from there.
But I had forgotten that the IBU has an irregular casing shape.
Still, a quick job to sort that when I get some time later.