How big a job is it to change the transmission top mount on a Felicia 1.3 MPi (1998)? Is it as simple as unbolting the old one and putting one of these (Felicia Upper Gearbox Mounting & Bracket - £19.99 : Skoda Parts, Jorily, The Skoda Parts Shop - Probably the UKs largest independent skoda parts supplier) in it's place?

Pretty much. You will need a jack to support the engine & gearbox during the changeover.

What would be the symptoms of a need for this mounting ?

The main symptom is a clunking noise that seems to come from that corner of the car. Also the rubber part of the mounting seems to have receded (see photos). This doesn't look right to me. I am aware that the same clunking symptom can be caused by the lower mount. The upper mount seems like a less intimidating task.

iirc there should be 4mm per side clearence between the rubber and the alu block.

the mount is rarely the course of knocking