Thanks for the pointer. It nearly worked, my issue was I couldn't find anywhere to select a size, rick mentioned he changed size from 61cm to 62cm after changing cost to £800, but I didn't see anyway of doing that, I could only select the large, extra large parcels but these were pre-sized, and were only available with Royalmail Parcelforce not the evri option I was hoping for.
I did ring ebay support, and they told me to select the largest option in simple delivery, and that when I created the label for that option, take the item to the Evri drop off point, and they would accept it even though it was oversize. The courier would check with ebay, and ebay would pay the extra cost, but not cross charge me or the buyer. Seems unlikely.....