Rob1n you sound very like me. DD is 18m now and has been in nursery 3 days a week since she was 10m old. She goes to MIL 2 days a week as I work FT (only way i get on mumsnet !)
I really understand where you are coming from - "but if I wouldn't buy chicken/chicken products labelled as being 'from Thailand or Brazil' for me & my DH, why would it be ok for my DS to eat it." Totally get that. I too couldn;t bf and I've only now really come to terms with it. We did BLW, which helped, but I was very worried about nursery.
However, whilst I would always aim for the best food, in my experience with DD it's the staff that really matter. Our nursery is in the grounds of a college and they use Asda for supplies, however they use a menu that has lots of carbs, fat, cheese, meat, veg and fruit eg cottage pie, macceroni cheese, fishcakes, but all the food is made on site by hand, they always have a hot pudding for after lunch and they have home made cakes etc etc. I have got involved myself and pushed them to switch to free range (which they did!) and gave them simple recipes for cheese muffins and oat/cheese flapjacks, which they have also implemented. they have really been willing to listen, which says alot to me.
Our baby room has a no shoes policy and the staff wear slippers. Once they're in 18m+ rooms they can wear shoes.
but, even though you will worry, if you feel at ease there, the staff genuinely care and your gut feel is yes, then go for it. As long as the food isn't pre-packed rubbish (or has too little fat - some nurseries are mistakenly following 'healthy eating' principles which are not good for babies!) your DS will be OK. i still hassle the nursery about potato cakes (which they buy in) as having too much salt but they only offer them once a month, so I think it's not too bad.
DD is very happy there, there is always a waiting list to get in, the staff have been there years and it is well known to genuinely care for the children.
You will always feel guilty working (and you'll feel guilty if you stay at home!) and prepare yourself for a few weeks of hell whilst your DS settles in initially - worst time of my life but I promise it will get better and you are making the right choices for your family.
Now DD toddles off into the baby room and waves me off with barely a glance she's so keen to see her friends and play with toys.