Ed Miliband showed once more yesterday that he is capable of getting under David Cameron's skin at Prime Minister's Questions by remarking on his close links to the City.

His "mates' rates" quip to describe the scandalous government handover of Royal Mail at a knockdown price was on a par with his earlier observation that, by lowering the top rate of income tax, Cameron was cutting taxes for "his Christmas card list."

The Prime Minister became so flustered over having his class loyalties so exposed that he claimed mistakenly that Royal Mail privatisation had been in Labour's 2010 general election manifesto largely written by Miliband.

It wasn't, but it is no secret that then business secretary Peter Mandelson, leading light of the Blairite undead, had tried to flog off Royal Mail in 2009.

Fortunately, the opposition coordinated by the Communication Workers Union and supported by other unions, campaigning bodies, many MPs of all parties and rural interests were able to frustrate Mandelson's efforts to gratify his parasitic friends in the City finance sector.

The tragedy of this broad coalition's inability to achieve similar success this time around is deeper than simply the financial loss.

Despite pledges given by Royal Mail to maintain the universal delivery obligation and common pricing across Britain, these promises will have the consistency of pie crusts once the privatised business seeks further ways to maximise profits.

Shadow business secretary Chuka Umunna has excoriated Vince Cable and his Tory chums for selling Royal Mail on the cheap.

He pointed out beforehand that Royal Mail is no longer losing money. It is a recently modernised, profitable business, which could have contributed substantially to the Exchequer.

However, Umunna, with the connivance of shadow chancellor Ed Balls and Miliband himself, ducked the challenge of warning the hedge funds, finance houses and other speculators that an incoming Labour government would take our postal services back into public ownership.

This would have stopped the privatisation express dead in its tracks and would also have enthused millions of voters to back Labour at the general election.

Leadership failure to do so may reflect Balls's determination to promise as few financial commitments as possible, in line with Gordon Brown's caution prior to the 1997 election, or it may illustrate the residual influence of Mandelson and new Labour.

It is to that lingering conservatism at the highest reaches of the Labour Party that Len McCluskey was pointing in his speech to the parliamentary press gallery lunch on Tuesday.

The Unite leader's cautionary tale that Labour could lose the election if it espouses a "pale shade of austerity" is soundly based.

The same applies to the warning that Unite, and possibly other unions, could disaffiliate from Labour in the event of a disastrous and utterly avoidable electoral defeat and look elsewhere to build a mass working-class party.

Miliband and McCluskey are due to meet today, which should enable both men to clarify their positions, but the Labour leader should not entertain the illusion that trade unionists are tied umbilically and in perpetuity to the party they set up.

Too many Labour leaders, Miliband included, have sought to win media support by demonstrating their independence from - indeed contempt for - the unions.

Labour is the only viable alternative to the Conservative coalition next year, but nothing is carved in stone for the future, especially if Labour contrives to secure its own defeat in May 2015.