Welcome to FixYa
To answer your question, "i am trying to open the drain plug to check for buttons etc but cant get it to unlock. is there a trick to it?
No not really. Just unscrew it counter clockwise,
The issue with these Asko
they can become clogged with items such as coins, hair-pins etc. that can actually damage the part.
Unfortunately once they are clogged to a degree removing the filter by normal means becomes difficult or impossible without breaking it.
The image I attached above is an example of a pump I had to remove and disassemble to clean it out so the filter could be removed/unscrewed, hence the need for the "worm-gear" clamps had to be used to replace the original "one-time" clamps. That though involves removing the access panel...
plus the tub bellows and drain hoses etc. pretty involved.
If you indeed break this part during your attempts to remove it, a replacement is available, Asko
original part # 88 010 69 and available at on-line
retailer "repairclinic.com" for about $25-$30USD.
Let me know how you make out or if you require further assistance.
Thank you for choosing FixYa
and good luck.