There's no cap. However there are cost limits on certificates to which extensions are given if justified. However grants will only be given for known work and all work is subject to a cost benefit test. So for example a cost limit might be £15k and if the solicitor applies for an increase to £20k they will need to be very detailed as to how the existing cost limit has been reached and what work is required for an extension to the cost limit and why the work is necessary.
If the work is for a new type of application (a change of scope) then again that needs to be justified and an application made for legal aid to cover that work.
Over £25k costs the certificate is subject to a high costs plan and carefully scrutinised by a special team with extremely detailed plans of work submitted