I guess both HCL and vinegar both aim to do the same thing = remove scale deposits that might be affecting pump action or valves. Vinegar is cheaper and I have no particular evidence of scale, so I'll try that first. Then I'll chuck some veg oil in to hopefully make the pump action slightly smoother. I'll also Vaseline the shaft to reduce the drip. At some point I'll work out the part number and take it to bits. Perhaps a job for when she is out of the water.