If it was me if ur serious about the house i'd go up and have a look at it, which may be a mission in itself finding it with vegetation cover.
Possibly i'm wrong but i wouldn't be overly corncerned about any reports or tests as u can add filtration systems to sort out most things.
When i bought the bit the sellers where absolutley useless and couldn't organise a test, in the end i bought it without a test as it had dragged on so long.
Wot i would do is take a bucket/container with u to measure the flow rate, that u can't change, i know we have had a bit of rain lately but ground still fairly dry.
If u know ur container is say 5l and fills in 'X' amount of time u can atleast guesstimate ur flow rate, and at moment be fairly low ish.
I dunno if there is a 'set' amount u need but if this lowish flow can fill ur holding tank overnight so full to brim inn morning u should be ok.
Althou th plumber that done our work said May was always the driest month for private water supplies, guessing extra demand from trees/grass growing fast as well as often being a good month weather wise.
Amazing how the flow rate can change but if u just look at th water flowing into the tank it just looks much the same
When we had our problems being communal all neighbours had to agree, it was like knocking ur head of a wall

trying to convince them we had a decent sized leak..
We sort of bodged the supply problem until a plumber could come out and do the works but still the 6000L tanks were empty 1st thing in morning despite enough flow going in to fill them easily overnight.
I had empty 5L paint tins hanging around all the different tanks so i could measure the flow rate at each in flow pipe to try and figure out where leak was.
They do say with a proper spring u have to be a bit cautious about digging about in it as u can 'lose' them