Pawel
it doesnt matter what the landlord thinks is acceptable you are the proffessional builder on this job and you are liable if it fails
the joists neede replacing and if the landlord wont accept your advice, you walk away and let someone else bodge the job up
you claim to have an nvq3 in electrical, if this is correct you would have been taught not to drill the joist in critical areas
you also claim to be a civil technician and have worked on large structures
I believe you were a site operative and werent responsible for the structural work, I also believe that when that job finished you went on a training course to learn electrical work, you cannot work here as an electrician as you do not have the relevent experiance or quailifications
there is a problem in this country in that the
rules pertaining to who is allowed to do what on site work doesnt apply in the domestic sector
anyone can do anything to a house with the owners permission, this has changed with electrical and plumbing, but anyone can call them self a builder and chop joists about
the landlord wanted someone to come in cheap and just bodge it, he doesnt care if it goes wrong as you are liable not him, in court he would claim that he didnt know what was what and you assure him you were a civil engineer and that you could repair the joist safely, you have no defence as you did go and repair the joist. the moment you start drilling the joists for bolts you became responsible for the structural strength
my advice to you Pawel is to go work for someone else untill you understand the
uk building regs and
uk building law
and do not touch any structural work untill you have been properly trained to do so