If I put a pulley at the top of my 70ft 80m vertical and use it to support the centre of a 80m inverted v is this going to cause any issues with the performance of the GP?
Most likely. You're planning to place the proposed vee right in the center of the near-field with each leg of the vee a harmonic of the vertical. When using the vertical, you'll have max voltage at the top of the vertical, right where the vee attaches. They will interact - how much depends on a number of factors, but like K2DC correctly pointed out "... you'll never know ... until you try it ..." AND "... unless you intend to drop the vee while using the vertical."
The same principle holds for guying towers. You break up (or cut to specific lengths) each guy wire by using insulators to prevent the guys from interacting with the antenna's signal.
GL, 73, Rich, K3VAT