Summary: can I knock through a wall that is already holding up an RSJ?
And they say there are no stupid questions.
Okay, details:
We recently bought a new house. It has 5 bedrooms but one of them is downstairs and has to be passed through to get to downstairs loo and to one of the doors that goes into the garden (this door also hosts the cat flap.) Currently being used as a WFH space / junk room. It does have a day bed in there too, for overflow guests.
This room is also next to the kitchen. We plan on knocking it through to make the kitchen larger so that it can accommodate our growing family, range cooker, farmhouse table etc.
We do have a separate dining room too.
Additionally, we have permission to replace the lost downstairs bedroom on the other side of the house with something larger with a shower room. (The property has lots of land so going outwards is no problem.)
So, the problem... the house is old-old and at some point somebody has knocked through the originally kitchen & scullery to make a larger kitchen. This means there is an RSJ running counter-to the wall we now want to knock through. I have attached some images and even a sketch. 
Has anyone ever had experience of this before and what did you do? If I can't knock through clean, could I have archways?
For some additional background, the window you can see is original feature maintained and looks through into our sitting room, not to outside. The sitting room and dining room are part of a two storey extension done 20 years ago.
Bedroom 5 is also an extension, albeit single storey and circa 1940. Originally put in as a small garage with indoor toilet (although we still have the original privvy in the garden!) and converted sometime in the 70s. Therefore the wall between the kitchen and bedroom 5 used to be an external wall and is definitely load bearing. We have some documentation about this wall dated around 1997, when it was repinned.
The space on my plan marked "garden" will eventually have a small conservatory. Or orangery, if you're posh. Or solarium, if you're from Florida
Kitchen is currently 5m x 2.7m. Knock through would extend it to 5m x 5.5m
Unwilling to go in other direction due to losing original window feature.