Thanks for the pointer on the pipe.

I've got all the parts now but just waiting for the weather and time to change the water pump. I have to do the work on the drive as the FF won't fit in the garage and I don't fancy having the bonnet in the service position with the wind blowing 30 - 50 mph as it's doing now and forecast to do until at least Tuesday.

If it goes on much longer then I'll have to get a local garage to do the job, but I hate paying someone to do what I know I can do myself.

It's still leaking but not much, about 2 litres a week, so I'm just keeping it topped up at the moment. 

Mine had the pipe leak. 2019 D5 HSE Lux SDV6 Indus
55 D3 SE
98 MX5 1.8 RS
gone 17 older variants of FFRR

Well, just to close this one down, I replaced the water pump today and it was definitely the pump.
Once I got the fan and drive belt off, the pump hub had about 3mm of movement in it and there was coolant coming from the pump's shaft.
Looking at the state of it I'm surprised it carried on as long as it did!

Anyway, after 5 hours of work the FF is running again with no leaks and both aux drive belts replaced into the bargain.

Thanks to all those who posted and offered advice.  

One other thing to add;

To get the viscous fan off I found a tool to hold the water pump pully which may come in handy for others who need to remove the fan. It's doesn't cover the whole of the pully nuts as they're recessed slightly into the front of the pully but it does work and fits the M62.

BMW Water Pump Pulley Hub Clutch Fan Holding Tool