John,
I come up with (10,000* 1.02^23 =) £15,769 and (7500+2500*1.02^23) £11,442, which are near enough to your figures to say that either set of numbers gives the correct impression.
What I think is more informative is to relate numbers to today's prices. That means the 3/4 portion of £10,000 pension that is currently worth £7,500 would only be worth (£7500*(1/1.02)^23 =) £4,756 at today's prices in 23 years time. In other words you would be (7500-4756 =) £2,744 worse off at today's prices. However few engineers of our vintage would only be on a Halcrow pension of £10,000. Those currently on £30,000 would be £8,231 worse off and those currently on £40,000 would be £10,975 worse off than they are today (pre tax).
On the plus side it would put those currently paying 40% tax on part of their pension into the 20% tax bracket!
What I have not checked is the 75% assumption. Also the assumption of 2% inflation could be wildly out if the £ devalues because we leave the EU. We could go back to the days of double figure inflation!