Is Dunlop foam: Enduro Protect, EN36-130, 50mm thick ok to use as mattress in co-sleeper. Clark Rubber will cut to size as I can’t find the right size replacement. It will go on top of the wooden board inside cotton cover. I know the density will be fine, but worried about toxins in this product? The latex alternatives are very expensive. Clark Rubber stated it would be fine , but I thought I’d double check. Thanks.

Red Nose general recommendations for a mattress are that it is firm, clean, well-fitting (no gaps) & flat (not elevated.)
https://rednose.org.au/article/what-is-a-safe-mattress
 
The following applies specifically to a
bassinet mattress.
“Firm, flat mattress that’s the
correct size for your chosen bassinet – the mattress should be no thicker than
75mm in thickness”

https://rednose.org.au/article/bassinets
Red Nose has no information in relation
to specific brands, types of fabric or specific “toxins”.

As mentioned the biggest concerns from Red Nose perspective is the fit,
firmness & flatness of mattress.

Soft mattresses that are not flat & firm have been implicated in Sudden
Unexpected Death in Infancy (SUDI)
How old is your bassinet?
Would it be preferable to place baby in the cot from birth?

Not sure how old bassinet is, it is a co-sleeper with mesh sides in great condition, but the previous owner has removed the foam and put an ill-fitting mattress in (5cm gap all around). The cot won’t fit in our bedroom.  There doesn’t seem to be any info on PU foams regarding VOC ratings etc.  So am I correct that material composition doesn’t really matter as long as it’s firm and fitted?

Red Nose research in relation to a mattress is only related to ensuring the mattress  is firm, clean, well-fitting (no gaps) & flat (not elevated.)

Some people do have concerns about what the mattress is made out of, but Red Nose is not aware of any specific type of mattress fabric that may contribute to SUDI (SIDS & Fatal Sleep Accidents)

You may want to check about the PU foams & VOC ratings, but Red Nose does not carry any information about this.