As others have said here, it's a questionable purchase. If the engineer says that it can't be altered to handle the fire regs, and if the floors are sagging, then it's not up to standard, and shouldn't/can't be marketed as 3 bed house when it's only a one bed. And if the landlord won't let you do an invasive check, then that alone should be sufficient warning. When you come to sell the property, then there'll be someone in your position questioning the price etc, and giving you the hassle.
Whilst you're solicitor can't comment on the engineers comments, they should be able to advise you on the legality of the property, and if you're not dealing with one of the partners, then you can ask one of them to take a look over the file to make sure you're being given good advice.
The structural surveyor is only looking to check for any defects, but if he finds any, then you use that to renegotiate the purchase price - did the SS spot the sagging floor, and non conformity to fire regs by any chance. The offer letter to purchase the property, should have been on the basis of subject to survey, as you don't want to pay top price, and then find you've got work to do. You've got a problem on you're hands, as the landlord may possibly drop the price to reflect the need to redo the beam and the joists, but will be reluctant to drop it by 40K. If you don't go through with the purchase - which I don't think you should - then he'll very likely give you notice to quit, so that you don't tell an subsequent purchasers of the problems.
A more detailed meeting with building control may help you find a way through this mess, and determine how legal the place is, which should then help you determine the value of the property in conjunction with an estate agent - remember the one you're dealing with works for the vendor, not you.
Basically, you're between a rock and a hard place.