In one area (IHT), I was following an 'alert' that HMRC had increased the interest rate levied on late payments to 3.5% ... and noticed that they hadn't changed the interest rate payable alongside any re-payments (it remains 0.5%)!
So I looked up the history of these rates at https://www.gov.uk/government/publications/rates-and-allowances-inherita... and was stunned (it happens to me all too often nowadays) to discover that everything changed radically back in March 2009.
Prior to that date the two types of rate remained in sync (whether 1% or 11% or something inbetween) ... which is how I remember things (i.e. it wasn't part of HMRC's remit to make 'a turn' on any overdue payments vs re-payments). But someone got greedy (Alastair Darling presumably) - and no-one has tackled the fiscal drag effect since then.
To save me more fruitless research, does anyone know what the equivalent position is (re rates for overdue payments and re-payments) in other areas - such as CGT, IT, etc?