You claimed that HM Treasury apply a blanket 60% uplift on projects. They don’t. They set guidance criteria to departments for approvals. The document you have linked is a Department of Transport document, not a Treasury one. Treasury expects individual department procedures to be statistically valid; the probabalised analysis I described is exactly how Optimism Bias uplifts are calculated.
Optimism Bias loading you would know that the applied bias is based on statistical assessment of previous projects carried out in Britain. For High Speed Rail, the uplifts are based on a basket of 43 global projects since there are only two UK examples. None of that basket of projects is anything like as complex as H2S.
The Optimism Bias adjustment process is probabalised depending on which stage the project is at and on what the authorities appetite for project risk is. It is categorically not a blanket uplift as you claim. So without knowing which appetite curve the price is quoted on, you are comparing apples with oranges.
Moreover, the guidance notes the very issue that very issue; the one I drew attention to before. Namely that different stakeholders use different cost measures to suit their agenda and purpose.
Personally, I’d question the validity of applying the methodology Optimism Bias loading to mega projects. It’s fine for a £10M road or a new station, but it’s validity on a program like H2S is very questionable.