Took it in today and they checked the TCU, reported that it is fine, bike didn't work with a known good TCU. So decided it must be the cable and having heard it was £29 for a new one thought it worth a punt. They sold me the cable but on getting it home realised it's marked for Brose motor and the SL uses a single unit harness that includes this cable but also the charging port, battery connector and motor connector too. All one part £95!!
Also found that the top battery mounting screw was completely seized in and had to drill it out (couldn't get it with external or internal screw extractors!) screw now broken off in battery.
Pretty sure this is down to bad workmanship by the shop. Think the cable/hose guide tubes need to run down one side of the battery and the screw holes are slightly offset to compensate and they put it down the wrong side and then forced the battery across and jammed the screws in!. I hate having other people work on my bike!