I’m assuming you have a lump sum (or series of lump sums) for prelims.
Answering the second part first, no the lump sum(s) cannot be reduced due to Contractor’s overrun. The overrun is dealt with separately, hopefully you have Option X7 in the contract specifying the level of damages for overrun.
In respect of the first part (the lump sums), under clause 11.2.28 the PWDD should include “a proportion of each lump sum which is the proportion of the work covered by the item which the Contractor has completed”. Therefore you should agree a reasonable profile for the payment of the prelims which may not be just £X/Y months due to peak levels of staffing etc. If you are in Contractor's delay then the profile will need adjusting over the longer period. Ultimately the full lump sum is payable.
Answering the second part first, no the lump sum(s) cannot be reduced due to Contractor’s overrun. The overrun is dealt with separately, hopefully you have Option X7 in the contract specifying the level of damages for overrun.
In respect of the first part (the lump sums), under clause 11.2.28 the PWDD should include “a proportion of each lump sum which is the proportion of the work covered by the item which the Contractor has completed”. Therefore you should agree a reasonable profile for the payment of the prelims which may not be just £X/Y months due to peak levels of staffing etc. If you are in Contractor's delay then the profile will need adjusting over the longer period. Ultimately the full lump sum is payable.