Problem replacing Euro cylinder lock

I need to upgrade our door locks. The rear doors' Euro cylinders were no problem - undo M6 bolt, turn key about 20 degrees, slide out. But the front door - also upvc - is different. It's a Yale fitting but there's no securing bolt into the cylinder from the edge of the door. Even thought there's a hole in the right place in the decorative trim, behind that a plate slides across when the deadlocks are engaged, so it can't be bolted that way. I thought maybe it was bolted from the other (hinge-ward) side so removed the double-glazing-like beading from the stiles/rails, but the whole inner panel of the door has been secured with silicone, and the panel covers where the bolt head would be. So if it was bolted from the inside they would have fitted the lock before assembling the door, which seems unlikely.
Any suggestions? Last resort is to get a locksmith to snap the cylinder and advise how to fix a replacement. (I had a half-hearted attemt at snapping but didn't want to end up with a bent but otherwise intact cylinder still fixed in place).
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Can you see anything through the hole that is in the right place but can't be used? If it big enough that a screw could be inside but just accessed via that hole? Could anything have sheared off?
I've never seen a Euro not secured by that middle hole.
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fred
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fred wrote:

It's too recessed to see - torch gets in the way of eye-line. The local locksmith suggested it might be an Allen bolt. I had tried feeling with various metric and imperial Allen keys but none gripped. I'll give that another go, otherwise it's brute force and mole grips.
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