Was the batting decent though? Trott had to bowl, Anderson got injured and Broad is woefully short of form. 400 on a flat pitch against 2 bowlers and 3 half-bowlers - and with lots of rain breaks which make it difficult for the fielding side to get into it - represents the bare minimum. They're a terrible side. England have been playing at half-capacity and still looked safe as houses.
It was in away conditions, English conditions don't usually suit well with Sri Lankans. They batted in overcast conditions early on and did fairly well. Most sri lankan batsmen should have pushed on with their starts and get a hundred rather than just the wicketkeeper though. Their batting should improve the longer they're their to get used to the batting conditions.
The opening partnership negotiated Anderson ok early on before his injury despite him looking very dangerous, and most batsmen played the other bowlers fairly well. A four hundred total should have been enough to apply some sort of pressure, but their bowlers bowled so badly, making four hundred for the english batsmen was a piece of cake.
Honestly, in the same position, I'm not sure Pakistan would have made 400

. And making four hundred in the first innings, with the poor weather and nature of the pitch, pretty much meant they were in a win or draw situation. So credit to them to at least achieving that.
There's plenty of time to prove themselves in the next test, at least they aren't starting one down in the test series and on the back foot. Always difficult for a touring side to claw their way back.
On a side note Cook, Trott, Bell look all in ridiculously good form. It's a bit daunting for the opposition.