The House of Lords has amended the AV referendum Bill so that the referendum would require the votes of 40% of voters to be binding.
This is extremely anti-democratic, but of course so is the Lords in general, so this is hardly surprising. (For all the Commons' faults, a similar amendment there was resoundingly defeated by cross-party consensus)
The obvious problem with a quorum for any vote where one of the options is to do nothing, is that if the quorum is difficult to reach, it's often more effective for those opposing change to not vote than it is to vote no.
40% will be difficult to reach. Only general elections get that high a turnout in the UK (and then not always in every constituency). If we assume, implausibly, a potential turnout of 60%, comparable with a modern general election, then if at least a third, but less than half, of voters oppose it, they can "win" by collectively not voting.
Lower the turnout further, and it becomes even less possible for a vote for change to succeed.
This does raise the question, of course, of what would be done in a situation where the turnout was good (for a non-general election) but not exceptional - say 20% - almost all of which was in favour of change because most of the status quo voters had stayed home. Parliament - especially the Commons, which voted against the quorum requirement in the first place - would then be under great pressure to pass further legislation to implement it anyway.
I think quorum requirements in general are a really bad idea. They're only useful in a situation where it's plausible that a minority interest group could call a vote with so little publicity and/or notice and/or accessibility that hardly anyone who opposed them knew what was going on, and so a minimum attendance to prevent this is required.
The rest of the time, they're either unachievable, leading to an organisation that can't make changes because it's impossible to get the votes to do it, or they're trivial - ten people out of thousands - and thus rather pointless.
In this situation, it's rather implausible that the referendum could be called without sufficient publicity or notice, and while some of the polling stations will be extremely inaccessible to some voters, that's not an issue that a quorum figure is going to fix.
So, essentially, if the Commons don't amend this back out (and the Lords have filibustered long enough on the Bill that they don't have time for a long argument without delaying the referendum) the vote is certain to fail.
That a narrow majority of the Lords care so much about the electoral system used in an election that they can't even vote in that they're willing to stoop this low to rig the result (rather than campaign for the result they want) is a strong sign that they need replacing with a fully-elected house as soon as possible.