Cargo 0813 electrical issues?

Hi guys, we've got an '87 Cargo 0813 horsebox, last weekend whilst trying to start, we noticed a burning smell from o/s of lorry and wouldn't start. Now we've got no power / lights to any instruments or ign lights, won't turn over, not even the interior cab light works! we checked battery, which is only few weeks old, with voltmeter, there's around 13.5v at the terminals, also checked that we've got power past the isolator switch and all good. Missus not happy!!