I'm delighted to say that Layla's results have come back positive, with the big lump being diagnosed as a benign leionyona between her bladder and her uterus.
So the plan now is to wait 5 or 6 weeks till her season is well and truly passed, and then spey her and remove the mass at the same time. The lump on her spleen didn't produce any results (which I believe is good news) so the vet will have a good look into that too during her operation and if necessary remove the spleen. Remember chest x rays didn't show any secondary tumours.
Also we now have a couple of potential foster homes lined up for Layla (a plan A and a plan B) so whilst there is still a long way to go, she will have a home to call her own whilst she completes that journey.