You have applied the correct calculation, although I came up with an adjustment amount of £963.86, due to the number of decimal places, that is 20% x 0,33 (a third) is actually 6.67%.

I usually have a 2 step calculation on an excel spreadsheet, with the first calculation listing the elements in column 1, the stated proportions in column 2, the base indice in column 3 and the latest indice in column 4.

The next step I calculate the increase in the indices as a percentage (L-B)/B for each element and then multiply by the proportion, totalling these to give the PAF (as a percentage)

The Net PWDD is then mulitplied by PAF/(1+PAF) to give the adjustment amount.

In your case it is;

Direct Staff - 2.00%

Labour - 6.67%

Material - 2.00%

Totalling 10.67%

£10,000.00 x 10.67%/110.67% = £963.86

Under NEC3 you also repeat the calculation where an Index changes (often an index is provisional before being firmed up at a later date), although this doesn't occur under NEC4.