Diamorphine is (legal, prescribed) Heroin. It's as effective as morphine; in fact, the diamorphine cannot itself ease pain - it has to be metabolised into morphine first. That process only takes around 30 seconds, however. It is more potent than morphine (when injected), because it is more fat-soluble, so a smaller dose of diamorphine can have the same effect as a higher dose of morphine.
It works in the brain, binding to opioid receptors, which reduce the perception of pain and response to pain. In other words, it doesn't stop the pain, but you don't know it hurts as much as it does!
Pethidine works in much the same way (although technically, it affects different receptors) but has different side-effects. The side-effects are different for each woman, though.
For me, I had Gas and Air and TENS for labour #1 - the G&A made me lose all muscle tone and sensation, but I could still hear. As the G&A wore off (within about 10secs) my sensation came back, then muscle tone. This happened at every contraction.
For labour #2, I had G&A plus TENS, but then got a positional difficulty, so pethidine was suggested to allow my body to 'relax' and give me more time (labour was progressing quite quickly). It was a horrible experience. I was fitfully dozing between contractions. I was aware of the contractions and they hurt! but I couldn't communicate the fact.
I needed the loo, but couldn't sit up properly, let alone walk. The midwife suggested I pop to the loo - they realised I needed a bed pan! I was in what can only be described as a 'Dissociative state' - I could hear the tv, but the sounds repeated in an echoey fashion. I would have periods of speaking gobbledigook, then suddenly 'come out of it' and see and hear clearly, but was very confused. It was horrible.
I have to say though, that 5 minutes before DD2 was born, I had a sudden moment of clarity, sat up and said 'I need to push this baby out!'.
labour #3 - Went back to my trusty G&A and TENS 