"As re-engined aircraft cannot take the 104 back-could mean bye bye Jag earlier than expected."
Why not?
There are plenty of 2,000 hour Jag airframes at Cosford, two of which have already been upgraded to GR3A standards. The conversion from GR1>GR3A costs little more than for a GR1A>GR3A, although the Cosford aircraft do also require a Major.
It would thus be a very easy matter to generate 20-24 virtually new Jaguar GR3As, perhaps allowing the type to remain in service a little longer, and perhaps taking the strain off the Tornado GR4.