Additional HB CTB Earnings disregard

I am doing a very historical calculation.  Does anyone know the date when the HB CTB additional earnings disregard started?  I think it was April 2002 and it was then £11.45 pw.

Many thanks

was it 4 october 99?

inserted by reg 5 of SI.No.920/1999?

http://www.legislation.gov.uk/uksi/1999/920/made

cpag handbook for 2000/2001 has it @ £11.25 ...

(cpag also reference hb circulars a7/99 and a41/99 if of use?)

Thanks, looks like it was introduced at same time as WFTC?  Amount is the same as the the WFTC 30 hour WFTC amount.