Hi, I have the same setup on my 90. I would be surprised if the bag is leaking, it is pretty tough and sheltered. More than likely it is the tubes or fittings. The pump is available from a number of sources.
Mine was inop and I thought about alternatives including pumping from the front, but wound up flushing the reservoir and pump, cleaning the terminals brought it back to life. If you do go with the front setup, you will need to source a reservoir with dual pumps.
Try these folks, I used their equipment when I replaced the washer system on my SIII. Nice guys, supply all sorts of OE systems but are willing to take time advising on things like old Rovers.... Also stock an almost perfect replacement for the big $$$ and low quality Lucas 2 speed wiper/wash switch.
AM Equipment Windshield Wiper Systems & Durable DC Motors