"There was someone on the TV yesterday saying that if there were a new leader elected, the opinion would be that to demonstrate they had the authority to lead the country, a General Election would be called. "
Yes I saw that. However no sane government would call an election (the procedural obstacles aside) if the opinion polls were bad.
New PMs do get a poll bounce though it is true, so maybe it isn't such a fanciful idea.
I am still not sure they would risk it. A lot of seats would fall I would have thought.