The part that matters: the question to ask is not whether a vendor is good but what evidence exists, of what kind, about which lots, from whom. Reputation is a compression of that evidence and it compresses badly.
Cost per milligram, worked honestly: a 10 mg vial at £34 is £3.40 per nominal milligram. If the content assay says 9.2 mg, that is £3.70 per actual milligram. If you then lose 4 µL of dead space per draw from a 2 mL fill across twenty draws, that is 80 µL or four per cent of the fill, taking you to £3.85. Add a £110 content assay amortised across the vial and it is £14.85 per milligram for the first vial of a new lot and £3.85 thereafter. The testing dominates, which is the actual argument for buying larger lots.
Mechanically, a defensible group-buy structure has three properties: the material is tested before it is split, the test is paid for from the pool rather than by the organiser, and the split is documented with photographs and a per-participant record of lot, volume and date. If any participant can reconstruct what they received from the records, a later dispute is resolvable. If not, it is not.
Independent testing costs have been stable enough over the past two years that amortisation arithmetic across a lot is worth doing before choosing a lot size, and the numbers usually favour a larger lot tested once over several small lots tested never.
The evidence you want is boring: the same result, from an independent laboratory, across more than one lot, over more than one year.