How to install a Flitch beam, advice please

Hi please could somebody guide me through the next part of a job I’m doing in my house. I got some advice from a builder friend but his company have sent him to a job in a different area for a month or 2 and I want to get on.

Basically I live in a cavity wall 1930’s semi house in which I have removed the chimney breast in our bedroom on the 1st floor in the past and I had a builder cap off the top part first with gallows. So now after having a builder check what I need to do, I removed the ground floor chimney breast and I have cut the slot for the lintel and new beam.

Room is 4.3m x 3.3m, he said the walls between our houses are probably 9” but could be 13” but to go with 9” for the purposes of this job.

When they built these houses, they ran the joists in the 1st floor/ground floor ceiling, along the longest room length so instead of towards the chimney breast (3.3m) they ran them across the room (4.5m)

Because they did this they ran 2x 6” x”2 butted up to each other across the joists to support them and into the chimney breast a few inches. My builder said this was a little unusual and this beam was just to take the spring out of the joists due to their length and that it would all stay up without it but might be too springy. Although it may all be alright without it he advised to support whilst removing the chimney and fit a flitch beam in its place and rest this on a 300mm lintel. He said just buy a 600mm lintel and cut it in half. He said order a Flitch beam 150mm x 10mm x room length +allowance for resting in wall at each end and that would give me something even stronger than what was there before.

I did not know what a flitch beam was before this job but he explained and I read up on the net too. So as I understand it, I order the Flitch beam with holes predrilled for me and bolt two new 6” x 2” timber beams the same length as the steel to it. He recommended holes every 6” which seems excessive to me and after reading on net it seems you ask for them to be not along the middle but rather 2” from the edge at alternating edges and not more than 600mm apart maximum. So if 600mm is maximum what should I go for in this job?

Also reading on the net it advises against cutting lintels as they are pre-stressed but there is a lot of arguments either way.

So now my questions, sorry I go into too much depth and worry too much about detail but it’s because I have not done this before.

So can someone on here confirm should I have holes every 6” on the Flitch beam 2” away from alternating edges or what do you suggest?

I want to cover the flitch beam in Oak so is it ok to drill holes big enough in the timber to accommodate the nuts and washers i.e. to recess them or will that weaken it or am I supposed to let them protrude, in which case how would you mount the Oak cladding?

What size stud do you think I should use for the Flitch beam, my builder recommended 10mm studs, does that sound ok?

The place you go to get the Flitch beam, can they make it up for you, i.e. fit the timber to it, or is that something you normally do yourself, I assume if they did it it would work out expensive?

Since this beam is only to stop the joist flexing and not major support am I worrying too much about stuff and should I just accept that this beam will be more than enough?

Will it be ok to cut a 600mm lintel in half and use it like that again because this is just to take the flexing out of the joists, or should I be concerned about this and not cut it, in which case what should I do as 600mm seems a lot of unnecessary cutting out for this job?

When I put the Lintel in do I cement it in place first or do I get the whole lot up and beam in place and cement with everything pressed up against the ceiling?

What mix of sand and cement do I use to cement the lintel in considering it is supporting?

I would love any answers to my questions confirming my methods or telling what else I need to know.

Sorry I’ve gone into so much detail on what is probably a simple job to a builer, but any help appreciated.