Shower corner joint opened up.

My 5yr old Gloucester has a joint opened up in the shower cubicle. This joint is a joke with just three shortened (10mm long) screw holding it together. The joint is vertical and the gap is from the floor up about 1.5M and as a consequence along the shower tray running full length forwards.
The gap is about 2mm to zero. I have pushed my damp meter in the gap and got no reading, so I assume it is dry.
Anyone else had this and how did you address it?
I have cleaned the joint with an alcohol wipe and frog taped to 4mm from the gap.  I plan to use a bathroom grade sealant. Is this the right approach?
HELP please.

Just spoken with AS technical. There is an aluminium strip in the end wall which the screws go into. On examining the screws closer it be can seen that they have broken. I have been advised to rotate the plastic insert a few degrees and put new (20mm long) screws in. Solution yes, BUT why have they broken?  Something is being strained here and it's not from an overloaded bike rack. CONCERNED!

Sorry to learn your discovery of a opening gap in your shower cubicle. Hope the advice given by Autosleepers will successfully close the gap and made further compliant with sealant. Not sure if a zero reading from a damp meter is likely, given there is always residual moisture up to 15% that is considered normal. Regards

Thanks. Let's see how long the screws last.

Same thing has happened to me and i found the screws were cut down and did not go into the metal frame work inside the outside wall.I drilled holes into frame work and put longer self tappers in and sealed the small gap left behind. I put this down to bad workmanship when built.

Agreed. I have to say that 'bad workmanship' is all over my unit. Put most right myself and made Marquis sort the rest. NOT what I expected from what I thought at the time time to be the flagship model of the UKs premium manufacturer. 
Just failed MOT due to rear number plate lights not working. Totally corroded. Poor design and incorrect light fittings applied, and will drill two drain holes in each. Now going to inspect all the rear light fittings for the same issue. I intend to correct all that need it and then spray all with ACF50. 
Hey Hum.