So the build has officially started - I should probably convert this to a Build log if that's possible!

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So the profile was joined using the self tapping M12 core screws, where the head slots into the 10mm profile slots and a hole is drilled though the profile to allow the T50 key to be inserted and tightened. This worked OK but I managed to almost break my wrist using a cordless drill and even a (cheap) impact driver struggled! Got there in the end and re-enforced with brackets.
I don't have any precision measuring tools at this point, but using the digital angle measure is 90.0 degrees on all angles and to 1mm across all dimensions.

Issue #1 - In my haste to add an extra piece of profile to my order (to have an L shaped gantry profile) the 2 profiles have different slot spacing. As far as I can tell the main issue is joining them. If these are joined simply by using the side gantry plates and socket head bolts to hold the pieces in contact is that a terrible idea? Could it be enhanced with some strong adhesive between the pieces to compensate. Really wan't to avoid wasting money on the extrusion if I can help it.. Any ideas?
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I've also placed the order with BST Automation - Thanks for the recommendations. All smooth so far, Fred seems very responsive and helpful. There are expected delays due to the coronavirus but it sounds like production is just slightly slower then usual.
Only linear motion components so far - 1610 ballscrews and 1605 for Z. 20mm Linear rails and Carriages. Flanged Type for X axis for stability as I'm using a wide plate.
For the blocks which will mount to carriages - Is Ecocast type aluminium recommended here? Quite pricey for 50mm thick block so want to be sure it's worth using this.
Also thinking of what you mentioned Jazz - adjustability. Would slightly oversize holes be the usual way to achieve this?

On electronics - I'm considering Hybrid steppers now, certainly on X due to the dual X motors and potential stalling. Any recommendations on models here?

Thanks!