Hi ezydriver,
When your flight is significantly delayed on departure from the UK, is it not possible to advise your handling agent at destination of your revised ETA and ask them to apply (at that stage) for a departure slot based on your new calculated ETD? In my experience, an early application for a slot revision has a much better chance of a pleasing outcome than one submitted very late in the turnaround process. If it is obvious that the return-leg slot is going to be unachievable, an early proactive response by your representative could result in you having a realistic revised slot awaiting you on arrival. Just a thought.
But beware the most common mistake. Crews can be over-optimistic in anticipating speed of turnaround. Maybe add on 10 minutes for the unexpected rather than *just* missing an over-ambitious slot and picking up a further 90 minute delay!
SHED.