Hey,
It's been a while and I realised I didn't do a follow-up post. I ended up not bothering with a MULF unit or aftermarket HU. I picked up a BT receiver which plugs into the Aux port, and I power it from the cigar lighter with a USB adapter. For £12, I thought this was the easiest/cheapest/best option.
Link here if anyone's interested:
https://www.amazon.co.uk/gp/product/...?ie=UTF8&psc=1
Cheers.
Stewart