Why is that, can't the get they train out of the way at Letchworth quickly enough?
I've only just started looking into it, as I'm getting increasingly fed up with standing in the cold at Cambridge North for a delayed train night after night! While these services were often delayed a bit in the past, there seems to have been a step change for the worse since December, so my first guess is that the new services running immedaitely ahead are at least partially to blame.
Eg. the 1724. From RTT - should arrive Letchworth 1759, moves off to the sidings 1803. Kings Lynn train then arrives at 1806. Seems a bit tight but doable.
Last Monday arrived 1801, moved off 1804 - the Lynn train was cancelled anyway.
Tuesday actually arrived 1802, moved off 1806 - delayed the Lynn train by 4 minutes.
Similar on Wednesday - but the Lynn was already 3 down at Hitchin, so not a massive delay.
Thursday arrived 1804, moved off 1807 - but the Lynn train was already 8 minutes late.
Very late Friday - looks like the Lynn went around it at Hitchin, already itself 4 minutes down.
So it seems it caused a delay 1 day out of 5 last week, but would have caused delays on 2 others if the Lynn train hasn't already been late for whatever reason.