I’d echo Neil’s comment regarding badly corroded engine earth lead; if you’ve a multi-meter and a charged battery (I’d take it out to charge it and then reconnect it) you can check easily. If it is a bad earth it’ll drain the battery to nothing in no time at all to the point the battery is beyond being able to be recharged.
Good luck - hopefully it’ll be straight forward and tracking it down is harder than the fix.
This is now fixed. The original battery was “killed” by the solar powered conditioner to the extent that it couldn’t even be detected by the charger at my local garage. Now the embarrassing bit; the new battery is fine but I didn’t realise that the negative connection actually has a switch on it (see pic). I must have turned this to “isolate” when I fitted the new one and once I realised and turned it back everything sprang into life. Thanks everyone for your help. Feeling stupid now!
Glad you got it sorted, just in time for the weekend too, going anywhere nice 
