Heard mainly in pockets of East Anglia, this is quasi-rhyming slang for "mental".
The Temple is the operative word, after an incident where someone who has the 1st name "julian" exhibited these characteristics. Now in ever increasing use among mainly younger people of East Anglia and beyond.
It is often used in the sentence
"He's gone Julian TEMPLE"
"It's all gone a bit TEMPLE" or other similar sentences.
Where "It's all gone Pete Tong" implies a situation has gone wrong, this indicates a person has taken on Temple "mental" like characteristics and is behaving strangely and or inappropriately.