Gary - in not having bought any model railway sets from M&S in the past, I am not at liberty to pass judgement on after sales of this type of product with M&S.
I do however, bought on numerous occasions from various model shop retailers locomotives, track and accessories and all purchases carried a form of guarantee against defects.
What the written word states within the guarantee, will not necessary be the cover you expected. The further you live from the retail source that you purchased the goods, be prepared for the hassle to have your purchase made good - I know, I have experienced it. In cases of purchases on the likes of eBay, you have no come back, sellers on eBay hide behind "let the buyer beware".