My daughter has just bought a sub for her GP1.4 Sporting 2007.
Before I start ripping it out, Does the standard B&M header unit have facility for low level output and a remote cable output.

Nope, B&M is a separate unit to the head unit

You may also find that it would be better to connect a sub and amp to a head unit, a header usually has an exhaust fitted to it :devil:

The sub has high level input, but i'd rather use the low level phono leads.
It would have been esier and less messy.

I'll wire the remote from the ignition, via switch also, and tap into the rear speakers then.

Probably easier, and safer, to tap the remote from cig lighter / power aux