Ooops - part number was indeed a typo - ERR3539 - Apologies.
You're not being a prawn at all - the so called quality of parts is quite shocking, and of greater concern, such a ****ing lottery it's infuriating...
In terms of repair, I've not replaced enough to not lots about them TBH - but I do have a failed one in the scrap bin, so I'll have a look and revert.... Always better to fix what you have I think - Although, I understand the electric ones are better - as they are only running as required, and I think this is the way I'd go when the need arises on my own D1's .....
Nope, never heard of Unipump. I doubt I'd bother with the blue box er, "offerring"
Found some more prices:-
Genuine Wabco £195 :-
www.ebay.co.uk/itm/383116218322
OEM quality with a four year warranty £92 :-
www.ebay.co.uk/itm/263977084666
5 year warranty one £64 :-
www.ebay.co.uk/itm/252248988522