The majority of cases have been reported in Sandringham Estate, Clumber Park, Thetford Forest and Sherwood Forest. However in 2010 cases were also reported in:

Blidworth Woods/Sansom Woods south of Blidworth, Notts (SK590525)
Laughton Wood near Gainsborough, Lincs (SK8400)
Sherwood Heath near Ollerton, Notts (SK645675)
Spalford Warren north of Newark, close to river Trent

In addition, cases have been reported in these other areas by dog owners:
Grantham
Ashdown Forest (East Sussex)
Pott Row woods (King’s Lynn)
Dalby Forest , North Yorkshire
Gayton Hall
Rufford Park
Holt (Norfolk)
West Oxfordshire
Kings Wood in Bawtry
Llanwonno (Wales)
Ballsy Pit, Kings Lynn
New National Forest
Horsted Woods, Norfolk
Abbeystead estate, Lancashire
Swardeston Common
Shouldham, Downham Market
New Forest
Bramcote Woods, Bramcote
Southend
Colwick Wood, Nottingham
Thieves Woods, Nottingham
Nettleshall Heath, Diss
Winterton Beach, Norfolk
Dalby Forest, North Yorkshire
Alexandra Palace Park
West Stow
Fritton Woods, Lowestoft
Pott Row Woods
Rendlesham Forest
Clowes Wood, Worcestershire
Morton-in –Marsh, Cotswolds
Holkham Beach
Burnham Market
Teversal
Potters Heigham, Norfolk
Carmarthenshire
Llanelli
Wokingham, Berkshire
Market Rasen, Lincolnshire
Guildford
Browns Bay, Islandmagee, Northern Ireland
The Devisdale, Altringham
Tongham, Guildford
Eccup Reservoir, Leeds
Gorleston, Norfolk
Epping Forest
Littlehampton, West Sussex
Charmouth, Dorset
Hunstanton, Norfolk
West Bilney Woods, Pentney
Foxley Woods, Bawdeswell, Norfolk
Somercotes, Alfreton, Derbyshire
Lincol
Boston, Lincolnshire
Market Bosworth, Leicestershire
Kingsteignton, Devon
Pembrey, llanelli
Eccleshall Woods, Sheffield
Blackwall Plantation, Carsington Water, Derbyshire
Lawn Park, Sutton Ashfield
Golitha Falls, Bodmin Moor

Didnt realise there had been cases in Bramcote Woods, thats my local park and actually the place where i'm taking the dogs today. Not many mushrooms up there though (or puddles or anything else I can think of that would harm them).

Thats quite a list isnt it? And includes places I visit myself (Thieves wood, Sherwood Heath, Clumber Park).