Roof not structurally sound after roof calcs. Where do we stand?

Bit of both, The roofer should have advised that a new roof covering can increase the weight and then this will cause tension on the supports, But however it is your property so this will be down to the homeowner but not a thoughtful roofer, And this is only timber to be added to the inside of the loft space so 1k should be more rhan pleanty to rectify.

The roofer he should guarantee his work

If I had inspected your roof I would definitely advised you of the situation, not ignore it causing more stress for the customer, as free advise is always my priority ,and shows good work ethics,w massey roofing

It depends on what kind of roofing company you hired. As a tiler/slater/or flat roofer, structural requirements are not within our scope of works. Generally speaking tilers / slaters (roofers) are not required to carry out any kind of structural works as this is a carpenters job. The tiler/slater will finish using tiles or slates and although materials differ in weight between natural or artificial slates, concrete or clay tiles, either choice will not require more or less structural supports. Building regulations should confirm this.