Good Morning all
Valdemar, firstly, thank you for the documents, it is nice to have and i appreciate your time in sending them to me!
Well i got my reply from the Polish company..... it is Lister Petter parts but his prices are prohibitive, 1 gasket set = 657-10432 JOINT SET - € 171,65

Speaking with S.E.P he sells on Spanish Ebay,.. often when i have bought items with import tax paid in final price to Ebay or Amazon they pass through Customs without issue as is already paid..... So i think, other than flying back to the HM land

i will try to purchase through Ebay.es... (the parts still come from UK)
To answer your points
PS1; I can believe there is no one left in Portugal, i have a plate on my engine from Original supplier, i will take a photo later to show out of interest.
PS2; If the gaskets are poor quality i will make my own, it is often the same with many 'quality' classic car parts, high prices for low quality... a shame!
PS3; No i have not opened the IP's.
When i first noticed the rising oil level, about 8 years ago we only used the Lister maybe once a year. so i would let it run out of fuel or after stopping the engine i would close the fuel tap.
it is only in the last 2 years we have been using her more often and turn off the engine immediately the water tanks are full...... and changed the oil more regularly than 250 hours!
Only in the last month i have seen one IP leaking a small amount from the oval window.
After changing the engine oil a few days ago, i cleaned the IP element through the oval window and oiled it, now it seems to have stopped..., externally.
(When i drained the engine oil I got about 8 litres from the sump and it was thin and a little smell of Diesel )
BUT this time i opened the air bleed screw on the this IP and i noticed very tiny brass pieces coming out with the fuel

I will report back later on my failure/success with Ebay
All the best
Alex
P.S i thought you might find it interesting, my Grandfather worked for the government after ww2 and was responsible for discovering rare art and restoring it.
He personally discovered many hidden painting on walls inside the Tower of London and used to visit Buckingham palace to personally restore many of the late Queens paintings.
Here he is in the Tower of London
https://www.alamy.com/medieval-painting-discovered-in-tower-of-london-a-close-up-of-a-specialist-craftsman-of-the-ministry-of-works-mr-henley-g-blake-intent-upon-his-intricate-task-of-uncovering-the-14th-century-mural-discovered-in-the-tower-of-london-during-repairs-to-the-structure-of-the-byward-tower-at-the-tower-of-london-it-was-found-that-the-main-beam-carrying-the-second-floor-above-the-gateway-had-painted-decoration-mostly-in-gold-this-decoration-included-leopard-of-england-many-times-repeated-in-gold-with-red-tongues-fleur-de-lis-and-small-birds-all-over-the-beam-february-8-1954-photo-by-fox-image471436367.html