I would play it like this;
Ask do they feel that 18 months is a reasonable amount of time for the product to last (taking into account the price and usage)
If they say no, happy days, if they say yes, explain that you would never have spent such an amount on their lathes if they didn't expect it to last at least that long.
Take names and quote the 'Sale of Goods Act', namely that a product can be expected to last a reasonable amount of time, 18 months is not reasonable.
If they still palm you off inform them that you will record a complaint with the Local Trading Standards office as they are currently in breach of contract.