Heat Flow meters

This probably isn't a DIY job - but knowing how knowledgeable this group is, I'm hoping someone can advise.
I'm involved with a local charity which rents out part of its buildings to another organisation, and also re-charges them for their share of electricity, based on "sub-meters" which we've had installed. That's the easy bit...!
We also re-charge for gas, which is used for CH & HW throughout the building. Because the main boiler supplies the whole building, we split the gas costs based on readings from three "heat flow meters" installed in the various pipes coming from the boiler.
These "heat flow meters" have now failed (they're about 10 years old). Repair costs are prohibitive, and replacements would cost well into 4 figures.
Does anyone have any suggestions (or a link) for a cheaper method of measuring the proportion of boiler output used by each "consumer"? Absolute precision is not necessary - an approximate split of heat consumption is sufficient for us to share the gas bill fairly.
Many thanks...!

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