Quite often in properties of this layout, there is/was a rainwater downpipe located just around the side corner, connecting to clay pipes (edited to add: just re-read your post and seen that you mention a rwp discharging to ground - same comments apply re mechanics of movement). Said pipes begin to leak (poor jointing, pipe fractures through settlement for example) and water leeches founding soils; soil under outer side of foundation becomes softer than on the inside, foundation rotates slightly.
Walls were not tied back at floor level in them there days as they are now, so the foundation rotation results in slight outward bowing of the wall. The entrance door and first floor window are close to the return, so there is little in the way of restraint to the wall end. Articulation takes place around the openings, as these create a weak spot in the front wall, leading to cracking like this.
Installation of replacement doors and windows can result in reopening of previous repairs, if they weren't done very well.
As long as the rwp is now connected to buried pipework to take the water away from the building, this may be old movement.
I suggest keeping a weather eye on it, before doing repairs, if the connection of the rwp to a drain has only recently been carried out.
Don't use a glass shard as a monitor, that "technique" went out in the English Civil War, for a variety of reasons, not least of which is that, by the time it breaks, it will be readily apparent that further movement has taken place ie completely pointless.
There are proprietary gauges available, either plastic gridded tell-tales, or studs and vernier gauge monitoring. However, you'd have to buy the whole kit for the latter, which is about £300, unless you know a friendly SE who'd purloin some from the office and take periodical readings for you.
Best way, as this is minor movement in the scheme of things, is to take photos every three months and compare with the previous. Tbh, now that the obvious cause (ie the rwp) has been remedied, I doubt that you will have any further significant levels of movement.
The worst that could happen is that you would need to install a single underpin block to the corner, just to transfer the foundation loading through the affected soil. This is unlikely to be required. The worst case scenario of underpinning and repairs to the superstructure could be claimed on your insurance, but don't forget that there is a policy excess, usually £1k and your property then goes onto their database as having a subs claim.
If it did need a spot of local underpin, personally I would quietly do it and wouldn't even bother with the local authority for BRegs: no names and no packdrill and all that
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You won't get anything from the installers of your doors and windows, it's nothing to do with their installation - at worst, it's merely exacerbated the levels of cracking, but is not the cause.
When it comes to doing crack repairs, this will involve hacking back the render and, I would suggest, installing Helibar across the crack. Then re-render, although you will find it nigh on impossible to match in, so it will be apparent that there has been a repair. An alternative would be to resin repair, so that the crack remains, but is only visible close up and certainly at a far lesser distance than a render repair.
All in all, don't panic, it's not a huge problem.
