Problem with the VRS

Started on Friday night the car was running fine, parked it up for an hour or two then jumped back in. The overheating light started beeping and flashing when i started the car. Checked the coolant it was a small amount in the bottom. Which was odd as it ran fine

Filled it up and checked it again on Saturday night coolant had moved a bit but nothing major. Then the car started running badly and went into limp mode. Checked it and coolant hadn't moved again from where it was, but a reflection of the light up at my BMC caught my eye. A pipe had melted on the silver bit on the right of the engine!

Spent all afternoon and changed the pipe after getting piping from a scrap yard from a Rover. So sorted that, but the cars still not boosting properly after filling the coolant back up. It's peeing it down outside so can't check my coolant, but it won't boost till around 2500rpm when it used to kick in at 2100ish and it's boosting at near on 26psi

Majorly worried, it's never once ran out of water so i know theirs no damage to the head and theirs no gunk on my oil cap either so the guy who recovered the car has told me the heads fine so thats a worry off my mind

Anyone any idea about the boosting? Could it be an air lock in the coolant system?