The clock starts as soon as you have the planning permission. You are then under strict timescales. Its important to check the detail set out on the Decision Notice.

If you don’t address conditions you may find yourself in a position where you have insufficient time to deal with them. If this happens your planning permission could lapse.

Generally no.

There are a number of approaches to dealing with this depending on what you want to vary.

Ideally you should address the planning conditions promptly upon achieving your consent.

In some situations you can reapply, citing the fact of an earlier consent.  However but much will depend on circumstances.

No.  The planning conditions are relevant to the grant of consent which runs with the land, save in exceptional circumstances.

The timescales are strict and inflexible.

Implementing the planning consent is a specific process, which can take place once you’ve dealt satisfactorily with the relevant conditions.

Where CIL applies it becomes payable upon implementation.

Much depends on the circumstances.

Why were the conditions posed as pre-commencement conditions.  Were you asked to agree the conditions before they were imposed? These are important questions and much depends on them and the reasons for their status.  

If you don’t address conditions you may find yourself in a position where you have insufficient time to deal with them.  If this happens your planning permission could lapse.