Ye gods … looks like my best friend has a rather nasty condition and I am wondering if anyone has any experience of treatment and costs etc so I can  make a decision x

I am sorry . We were un this position a couple of years ago and decided not to go ahead
The vet was very good and told us what to expect.. it appears that generally dogs do not get the side effects we do that may vary a bit regarding which type they are fixing. 
The oncologist was excellent amd explained it all in great detail including costs
We decide not to do it and let things tale there course
Fortunately he We are 2 years later amd she is fit amd well on the outside at 12 except for some hind weakness .hopeful you have a knowledgable vet or ask for a referral to a specialist
Costs were 3 k at least amd fortunately she was insured

When I felt a lump in the neck of one of our hounds, who was 12+ years at the time, I had my vet check and do a biopsy which turned out to confirm lymphoma.  Given his age, we opted not to treat and he lived a further 6 months in 'perfect' health to the point I had gone into denial.  Sadly on the final day, he didn't turn up (from the garden with the others) for his second meal.   I found him collapsed out there.  We took him to our vet who gave him a shot (?) to see if he could be revived.  The next morning he was no better so we sadly had to give him peace.  It hurt but at least he had a good 6 months with us.   Yes dogs are said to tolerate Chemo better than we humans do, but we did what we felt was right for our dog.

I think each dog and their illnesses has to be considered individually, with the aid of a trusted vet.