Here you go Naki
Its an Australian link but I sure a bit of googling would turn up the same in the UK.
http://www.penriteoil.com.au/products/coolant-test-strips
Don't use rainwater. Clean rainwater is mildly acidic typically ph 5.6 vs pure water at ph 7. This is due to atmospheric pollutants like sulphur dioxide. A 50% coolant/demineralised water mix will be alkaline somewhere in the range of 8.5-11 depending on the type and brand of coolant. So aside of the contaminants in rainwater, you're going to have to use a higher proportion of coolant in the mix to get the ph value up. De mineralised water is much cheaper per litre than coolant!
And I don't normally get this amount of time off. Have had a wee problem with the HR department and not taking enough annual leave so have had to cut out a large chunk.
Cheers
Steve