Right then... over two years have passed since I added to this thread, during which I've dealt with a serious medical problem, almost all of which I'm relieved to say is behind me now! Back to the Skoda Octavia..... I didn't have the opportunity to explain how I removed the starter before, for the reasons above, but I can now do so, if this helps any other owners.
1/ In the engine bay, remove the split pin from the gear linkage on the steering column & detach the linking rod, tucking the rod safely out of the way.
2/ Unscrew the vacuum pipe from the carburettor to the distributor at the carburettor & tuck this away too.
3/ Remove the metal heater pipe running from the lower radiator hose to the rubber connection to the heater matrix at the bulkhead (2 Jubilee clips)
4/ Unscrew the nuts and bolts holding the exhaust manifold to the down pipe, allowing the down pipe to be pushed lower, to assist in the starter's removal.
The two 17 mm nuts are now more accessible, at 1 & 7 pm positions. The lower one's just about accessible from under the car using a short reach spanner and the higher one's best removed by an open spanner, lowered between the bulkhead and the engine block. I eventually managed to loosen it a little and then used a 17mm (with two extensions!) to allow me to slowly unscrew the nut. Once the nuts are off I tapped the starter motor with. rubber mallet & it detached from the block. I was the able to wiggle it out.
In a day or so I'll continue the saga of obtaining a new starter & fitting it. It's all good news!