Hi all.
Thanks for your comments. I thought I would let you all know what transpired and the lessons learnt that may help somebody.
The insurer was Santam with whom I insure my cars, house and household contents. I had a broker but am not so sure it added value. The bike (BMC Team Machine SLR01 with Dura-Ace di2) was insured for R125000 (probably the first mistake). Value new is c. R180000.
The original assessment by Carbon Bicycle Repairs in Durbanville was to repair the frame but as that voids the warranty (thankfully BMC warranty extends to 2nd owner) I refused this offer, After getting Riders at Work involved (they recommended a replacement frameset on warranty grounds), Santam eventually agreed to replace the frame. They refused to pay the R90k that an equivalent frame would have cost (remember at that point to my knowledge there were no exact replacement frames available). RAW mistakenly sourced an SLR frame (value R36999) and Santam wanted to settle for R40k, which I again declined as it is the lower tier frameset. Thanks to information received on this thread, I sourced an exact replacement frameset from Abloc (previous agents I think) which is being couriered to my local bike shop as we speak. Final settlement value is c. R60k (frameset was on sale, plus rebuild and courier costs).
This whole process has taken 6 weeks with lots of back-and-forth and frustration, all so Santam could save R30k.
Lessons learnt:
Check your policy and the insurers protocols (repair vs. replace) BEFORE you need to claim and get it in writing if possible. My broker did not explain any of this to me and the policy schedule is not specific either.
Insure your bike for new replacement value, not the price you paid 2nd hand (or current "value"). Again, my broker did not give me advice on this. I spoke to Cyclesure and this is what they insist on. I believe it will give you a better bargaining position with all insurers. Premiums will be higher but this is definitely a case of getting what you pay for.
If you buy used, try get the original invoice from the seller. Santam insisted on this but eventually gave in. They want to be able to verify the value of the bike as they think all their clients are trying to rip them off.
I'll be switching my bicycle insurance to Cyclesure as I do not want to go through this frustration again.
Hope this helps somebody.