@Sue I have finally got on top of my low pH
Initially I put the suggested dolomite in the water conditioning wheelie bin, it arrived as a crushed coarse grit/powder sold for soil conditioning. The immediate result was very cloudy water which initially concerned me, but after a few days it had settled but still coated all the surfaces with a whiteish layer with the bigger solids on the bottom. The water was however clear enough to be used. The kH had increased a bit but it was still well below 1dKh, probably because the tap water is only just acidic.
I had originally planned on putting a bag in the sump with the thought that as the tank water became acidic the dolomite dissolution rate would increase and balance out the KH / pH. The cloudyness put a stop to that plan and a retest of the tank water at the end of the week showed the pH was still falling (4.6 if I believe the pH probe)
I left things like this for about 6 weeks to see if there would be gradual improvement - I don't think I was being cruel because they are South American Ciclids, in good health and have being growing rapidly and successfully breeding in these conditions.
However, this still seemed too risky to continue and I had a fairly major plan b which has I think solved the issue. [Yes everybody, I know I could constantly use a commercial buffer but that would be cheating
]
I had originally put a hidden drain pipe up through the floor when I installed the tank to make water changes easier, so I have converted the tank to a trickle system. I have a bank of three 12" water filters and fitted carbon blocks in all of them to remove chlorine (lot of carbon, tiny flow, no Chloramines) The water feed enters the tank at about two drips per second and at this rate does a 100% water change over the week. The pH is now stable at 6.0 plus as a side benefit the Nitrates levels are flat and very low so I'm sure the fish will appreciate it.
One thing I immediately noticed, a 100% change by drip is not the same a weekly partial change. It is just a constant very slow dilution so you don't get anything like as efficient Nitrate removal as a big change - otherwise my NO3 levels would be falling (a 40% weekly change would normally keep this tanks NO3 levels flat) Using this this logic I would assume 'everything else' that a weekly change does for water quality is also less effective and I think I will continue with smaller bi-weekly changes with the dolomite flavoured water.