third party subwoofer wireless connection

This won't work. Sonos does not allow third party speakers (let alone subwoofers) to connect to their speakers. But, since the sub connection on your Amps is active (I do not see that you use it for surround duty which would disable the sub output), maybe something like this will work: How to Make Your Subwoofer or Speakers Wireless| SVS (svsound.com)

Thank you that could work although I have read a few threads of the wired amp port not working!
 

also slightly worried about a delay doing it that way?

The sub output on the Sonos amp is turned off when the amp is used to power surround speakers in a home theater setup.  That does not sound like your use case.

As far as delay goes, that does seem to a be a legit concern.  There would not be any delay on the Sonos side of things, but it does look like the SVS device does introduce a delay with it’s 2.4GHZ wirless connect.  The expectation is that your AV receiver should have an adjust for this, which the Sonos app is not going to do.  Perhaps not an issue if your sub is place very close to the amp, but then, why use wireless in that scenario?

If these ceiling speakers are not part of a SONOS surround room, you could use a line level wireless extender connection to your subwoofer’s line level input. A consideration would be potential latencies in the extender. If the extender is intended for this sort of application I expect that any latency would be low.

Thank you - Yes the speakers were wired into the ceiling and are a long way from the amp, so running a cable to the subwoofer is not really practical 

Presumably I can add a wireless Sonos sub in my configuration?

Yes, you can.

Thanks everyone - not sure how much difference the sub will add!? 
 

a lot to spend as an experiment!

Might just be better to get a Sonos 5 speaker and abandon the ceiling speakers!

Worst case Sonos offers a generous (varies by country so check) return policy.