Thanks for the responses. I knew my MN Zoflora would come in handy one day!
I bought a few cheapie nighties from Ebay and BonMarche, and started off binning them if too rank. But realised that perhaps I need to try and salvage them first. I've washed a few normally, but am convinced I can still smell it. So I might need to buy a few more so that I can have a few at a time soaking in a bucket.
I'd go for the reusable nappy wash too. 60 degrees cotton wash with a pre-wash, colour powder. You would probably need cotton nighties, and no reds, purples or pinks or they would run. I have done this for nappies and DC clothes for almost 5 years, occassionally whites have needed a second wash.
I would usually suggest clothes a size too big as cotton shrinks at 60 degrees, but this may not be necessary if the nighties are roomy.
yes to bio powder. vinegar or dettol (the normal clear disinfectant) in the softener compartment also helps. don't use softener, that locks in the smell. wash as warm as possible. dry outside if you can.
For my Nan's wet knickers/clothing I put a slug of zoflora (clean linen scent) in the drum of washing machine before putting clothes in and then another slug into detergent section in the drawer. This with a long wash, plenty of washing powder and lenor febreze scent tends to save most things. I like you was having to throw a lot away due to lingering smell but this little combination seems to do the trick.