Personally, I would insist on genuine parts. That said, TBH, it's a discussion which needs to take place before policy inception. IMO, it's a salient enough point, but the onus is on you to read (and understand) 64 pages of smallprint!
ICOBS 6.1.5 (is a requirement)
"A firm must take reasonable steps to ensure a customer is given appropriate information about a policy in good time and in a comprehensible form so that the customer can make an informed decision about the arrangements proposed".
The whole process is designed to favour the insurer; they get to nominate a nationwide repairer, one who will give them a really good deal and in doing so becomes their "approved" repairer (which basically means the 'glass helpline' diverts to their call centre). This repairer might be giving them an average invoice deal.
Back to the failed attempt at repairing the chip - did the repairer make any attempt to cool down the screen before he started? What kind of equipment was he using?
A bit about genuine versus aftermarket glass
here. There's also the issue about who fits it. Whilst a Mk5 isn't the hardest job to do, there are so many fitters getting them wrong (and getting away with it!). Problems such as, damaging the interior A-pillar covers; scratching the paintwork under the trims; not removing the wiper cowl; wind noise and rain sensor not working are the most common issues. I was called in to investigate a problem on a Mk4 some time ago (the issue was wind noise and water ingress). After removing the windscreen, it was quite obvious that the could-not-give-a-feck-balloonhead-fitter had not removed the wipers or the wiper cowl:
Oh, and note that a lot of R32s will have blue glass.