Does the section 20 have a time limit?

In December 2015 I was notified, under a section 20, that work needed to be undertaken to the car park at a cost of £497.38 per flat (24 flats in block) which we paid in December 2015. Part of the work was completed in 2016, but work ceased due to additional work that was needed.

The cost of the work completed came to £303.75 per flat so the management company are holding £193.63 per flat retention. We were notified in early 2017, again under a section 20, that the additional work to repair the car park will cost approximately £2500 per flat, but we haven’t had to pay that sum yet. We have not received any notification when the work will be undertaken.

Our question is: are we entitled to expect a refund of £193.63 which the management company are still holding from their original cost?

Also, how long from a section 20 being served do the management have to to carry out the work?

Does the section 20 have a time limit?

Many thanks

Joe