I have recently moved to "PlusNet Broadband Pay as you Go with included 1GB bandwidth". I believe this limit of 1Gig is not restricted anymore and unlimited download limit is available on this account at no extra cost. Is this true ?
No it is not true. The PAYG accounts come with 1GB of data transfer as standard and you can either pre-pay for additional bandwidth or pay for what you actually use in 1GB increments. Pre-paying is cheaper by 25p/GB I think.
You are probably talking about broadband Plus (also costing £14.99/month) which has no usage limits but is no good for P2P or binary newsfeeds as it is trafic shaped (i.e. speed limited) for that type of traffic and priority is given to web & email traffic.
The alternative is broadband premier @ £21.99 where there are no (published) limits and much less traffic shaping (there is still some for P2P/binary newsfeeds) but going over 150GB will mean PN will contact you to reduce your usage.