An update.
I thought the pressure recorded on the RO (we’ve got a Pre Freedom engineered one) was the actual pressure coming out of the pump so I assumed we’d lost pressure. What I realised when I turned the valve to waste was that the pressure dropped to zero or thereabouts which gave me a lightbulb moment. The issue wasn’t the pressure in at all but the fact that our pre set valve was chucking too much to waste.
We bought a new valve without the Pure Freedom presets and we’re now about 65/35 waste to pure and getting around 250 litres an hour with pressure at 100 psi. We were getting 80 litres an hour so three times more now.
So a new pump, new membrane, new pre filters later, the fault was a £9 part. A good lesson though for me and my ops manager.