Thanks for the pointers everybody.

Ralph was correct in that I didn't know that jackd was a virtual package to cover both as the libraries are meant to be (on the whole at least) compatible so I admit that always confused me a little in the past. Makes sense now.

Thanks for pointing out jack_control, I should have searched that first rather than brute forcing it with the ugly commands I know.

Well it seems to stop and start with no issues with jack_control so almost certainly the issue lies with qjackctl.

You all say it's working fine for you. As I said it was for me until today, well last time I would have had to use the function before today was roughly two weeks ago, but it has worked with no issues in 18.04 between installation a few days after release (I got a new SSD so didn't wait for the point release like I usually would) up until today. So it's a new issue with a previously working package, not something that has always been like this on 18.04 for me. When did you last do an apt upgrade?

Thanks for your help, Dale.