Good to hear that you had some success!
Lets just consider this from a comercial point of view.
Called out to look at £15k worth of machine today - its older than I am but mechanically sound.
The machine is old and uses obsolete parts, did it cost £15k in 1975 or is it worth that today?
Whichever it is, the machine is obviously doing useful work or they would not want it repaired.
How much would a replacement machine cost?
not going to be a cheap job for the customer though
But what are you comparing it against?
This machine is obviously worth more than the average TV, it presumably does something profitable for its owner, and they can offset the repair costs against their tax bill. (Assuming this is not some shady outfit!).
So, your bill should represent money well spent for them.
On the plus side, the company they were going to get in would have charged ten times what I'm charging
If you know that for a fact, you should maybe be charging twice as much.
They should still be happy and your business will be more profitable.
Well done!
JimB