Just to say that I did indeed restart sql server agent (tecnically speaking, as it is a cluster i took the sql server agent resource offline and brought it online again). This worked - that is to say sql server agent did starting running jobs again as per the schedules. As i mentioned in the original post i was also able to get an individual job running on its schedule again by running the job manually, seemingly bringing the job or its schedule back to life in some way!

Anyway, as it's a non-production system the feeling here is that i put this one to bed and keep an eye on this cluster and the others on the same virtual machine host. If i ever see this again i shall log a call with MS.

Cheers all.