W639 Washer bottle grommets

[Kiwi Viano Owne]

Hi all. My wife's V350 Viano has developed an annoying problem where it drains the washer bottle within a couple of weeks - I have traced the leakage problem to the grommets that go into the bottle to seal the two pumps (front and rear screens). I have searched all over the internet for replacement grommets, or at least the part number for them, and so far have had no luck - I have been able to track down water pumps c/w grommets (p/n A2218690121 I think).
Can anyone help with the following:

• are the pumps for the front & rear screens the same?
• are the grommets for the front & rear pumps the same?
• what are the part numbers for the grommets?
• are they available on their own? (without the pump)
• where are they available from?

Many thanks in advance.