This is interesting because it's based in Southport (can't spell Houghton Street on its website) and there is another Nationwide business in the town (apart from the building society) which deals with the buying and distribution of fruit and vegetables throughout the UK.
This is a highly respected multi-million pound annual turnover company, Nationwide Produce Ltd, which is the largest independent business of its type in the UK.
Re the telephone numbers on the letter you mention. Are these the same as the "new" company and, if so, do they work?
Same for e-mails? Any responses?
As I live in the town I could perhaps get the local paper's editorial staff to undertake an investigation.