THE general election is wide open to fraud, thanks to the headlong rush into postal voting, the Government was told yesterday.   Ministers have been urged for years to introduce safeguards against cheating but have refused to change the law, according to John Owen, Birminghams elections officeThat, said Richard Mawrey, QC, sitting as the election commissioner, is something that would be appreciated by anybody who is minded to go cheating. The first election court to investigate fraud for morem than a century has been convened to hear petitions complaining of widespread corrupt and illegal conduct which helped Labour to take first place in Birmingham City Council s elections last June.     Voters described how they were persuaded to hand their completed ballots to Labour canvassers to be delivered to the election office. The votes appeared to have been opened and changed, using correction fluid, in favour of Labour before being counted in the poll. An election manager testified that the office received phone calls during the campaign asking whether votes altered with correction fluid would count even if they arrived in sealed envelopes that had been slit open. Callers were assured that they would be valid.      Democracy?They want democracy in Iraq.What about democracy in the UK? Tell us something we didnt know last june.