Hi @smiley , do you have an ohms reading for the element (heater circuit) at all? That might give a clue as even with continuity, it might have a short and therefore draw excess current, throwing the trip. 
Of course any resistance needs to be tested without power on the unit. 
Failing that, is there a way that you can monitor the voltage when the unit switches on?

Creda troubleshooter 

Thanks, yeah, I tested the continuity on each element as well as resistance that came up at 28.3 on each they're in parallel not series so hopefully individual readings and not all three. No trips were blown and the switch fuses on old and new switches were still good. And like I mentioned I did wire the supply flex via a plug and extn lead and element 1 started to warm immediately. I'm just flummoxed 'cos I was a chef not a sparky..... don't know where to look next...... they are 20 odd years old. It's the daughter's bedroom and might be able to check supply when she's back at work in the New Year, there's power as far as the switch and continuity through the flex.🤪‍

Hi folks, thanks for all your suggestions. Turns out thd initial problem wasn't the machine but the *edited by moderator* poor replacement switch I'd bought from Wickes, double pole and it wasn't engaging without a fiddle, so I got a new one. It ran all night then I discover.... the thermostat is knackered nearly, burnt out the new reset switch so I got to get on of those. Sellers show the same model then tell you you may have to accept the alternative model, which looks as if it doesn't even fit to the same fixings.... and you can't specify your own model..... why show it if you can't have it. Anyway.... switched off 'til New Year now, will try supplier of reset switch.

Happy New Year everybody.