a SURVEY of some of Britain's leading gardening experts, including the former Gardeners' World presenter alan Titchmarsh, shows less than half use peat-free compost, .  The results of the survey by the RSPB wildlife charity look set to reignite the debate over peat extraction, which has raged for 20 years. Last week, alan Titchmarsh was criticised by Friends of the Earth for arguing in Gardeners' World magazine, that sometimes there was no alternative for gardeners.  The RSPB wants a Government tax on peat products - such as ?1 on the average bag of compost - to encourage people to buy greener alternatives. Conservationists say extracting peat for use in gardens in the UK, much of which is from Ireland and the Baltic, leads to carbon dioxide emissions and damages natural habitats and wildlife.  according to the survey of 27 gardeners, broadcasters and writers, two-thirds backed the Government's aim of phasing out peat products. Most also thought consumers did not know enough about the environmental impact of peat extraction to make an informed decision.  alys Fowler, former presenter of the BBC's Gardeners' World and The Edible Garden, who does not use peat, said a levy on peat products would make "a real difference". She said: "I think there needs to be more education about the benefits of the non-peat compost - it can be just as effective as peat.  "as well as reducing carbon emissions and protecting peat bogs, peat-free compost is also a great way of recycling our waste. Just as we do not have endless peatbogs, we also do not have bottomless landfill sites."  Writing in Gardeners' World, Mr Titchmarsh defended his green credentials, saying he mostly gardened organically, conserved water and supported wildlife in his garden. But he said he used peat compost for seedlings that would otherwise fail to thrive. Other gardeners who still used peat include Noel Kingsbury, Christine Walkden, Helen Yemm, Bob Flowerdew and adam Pasco, editor of Gardeners' World.  The RSPB said some of those who responded to their poll were concerned that the lack of decent and widely available peat-free alternatives was a problem for gardeners keen to make the switch from peat.  Olly Watts of the RSPB said: "Peat extraction is a significant source of carbon emissions and causes destruction of natural habitats. Many gardeners understand this and are redu