Last night I found and tried installing VocalizerEx and set that in the Android settings as the default TTS Engine.
In Sygic voice settings a new option appeared - English UK System (Advanced Voice).
When I set that it sounds like the VocalizerEx voice is being used (it even increases in speech speed in Sygic if I make the adjustment in the Android settings).
I am going to try it later today if I have time (I have to drive 5 miles to get to a place where there are Street names to test.) and will report back.
If this works then I'll be satisfied as it is only the Street Name announcement I am after and I'll use whatever TTS gives me this with least problems for the next few days as I am away visiting different places until mid week.
However I will still clean out Sygic 16 and as Chambers suggests, install V15 and try loquendo again later in the week when I have the time, keeping vocalizer as a fallback option (if it works).