The 15% rate vanished before it began.
The ERM tried to hold participating currencies within agreed ranges. Rates had already risen from 10% to 12%. At 2.15pm, the Government announced 15% for the next morning. At 7.38pm, sterling left the ERM. The next day's rate was 10%, so the promised 15% never reached a borrower.
The whiplash exposed a deeper conflict. The Bank later wrote that defending sterling's ERM position constrained the scope for lower UK rates while Britain's economy was weaker than Germany's.
10% at the start. 12% applied. 15% announced. 10% the next day.

Make a canal walk accessible to every family.
Moor Time's families, staff and supporters will walk, jog and wheel from Saltaire towards Leeds on 26 September. Everyone can choose a distance from a short hop to a 14-mile finish. Funds will adapt its Ilkley venue so disabled young people can use the holiday and Saturday clubs while family carers take a break.
