Not being familiar with the relevant law, I’ve not been able to find a definitive answer. However, I did find this:
http://www.parliament.uk/documents/commons/lib/research/briefings/snpc-05142.pdf
In 2005 the Labour Party manifesto stated that the party remained ‘committed to reviewing the experience of the new electoral systems’ and repeated the 2001 statement that a referendum remained the ‘right way to agree any change for Westminster’.
The implication here is that while a referendum may have once been the right way, it wasn’t and still isn’t the only way that a change to the electoral system could be effected.
This leads us to a nightmare scenario – albeit a remote one. But we are in uncharted waters, with some of the most calculating and devious bastards in history squabbling for the helm of the vessel.
Others have written that a Lib/Lab coalition with Brown, or A N Other unelected leader, would so enrage the electorate that they could never win a referendum on voting reform and would thereafter be savaged at the next general election.
So, suppose the Lib Dems went into a coalition with Labour, Postman Pat installed at the helm.
They all want PR. Once they have it, their ‘progressive consensus’ will be de-facto obtained.
If they don’t need a referendum, why on earth would they hold one? Remember the Lisbon treaty?
Am I missing something here?
AJ
UPDATE:
I’ve not considered the House of Lords, but I do know about Labour’s use of the Parliament act.
Which leave the Queen. Could she actually do anything?
Al Jahom's Final Word 
“Am I missing something here?”
We need a /public/ vote to get PR as many Labour backbenchers would vote any such proposal down in the HoC.
You have to remember governments only hold referendums if they believe they will get the “result” they are looking for. If they don’t believe a referendum on PR would give them what they want then they simply won’t hold one ….
When I was younger, and flirted with politics at college and university I used to think PR would be a good idea. As I’ve matured (well to some extent, I’m still childish in some ways lol !) I’ve become more pragmatic and I can’t help thinking PR would just result in nothing ever “getting done”.
Do they need a referendum? IMO Yes absolutely.
One of the many things that ticked me off about Bliar and Liebour was his repeated use of the ‘enabling act’ for matters for which it was never created or envisaged. ie Hunting, Kiddy Gay Sex etc.
I will be less than happy to put it mildly to see a Tory led governme3nt with exactly the same Cavalier disregard of democracy. It’s a slippery slope from this to illegal wars etc.
I expect better from ‘Call Me Dave’.