can someone name some of the nearest chalk streams to south wales please

The rivers of the Cotswolds are your best bet although not all strictly chalkstreams, most running off limestone. The most accessible in terms of tickets is probably the Glos. Coln with the Bull Inn stretch at Fairford a good bet and there are also the Bristol Avon, Leach, Evenlode, and Windrush plus their tributaries, including the very good Box Brook near Bath.

For other chalkstreams and the next nearest to the Severn Bridge you need to head down towards Salisbury Plain where you get to the Wylye, Nadder, and other tributaries of the Hants. Avon and the main stem itself but fishing is hard to get unless you are a member of Salisbury AC or use their day ticket stretches. The Avon Springs fishery has stretch of the Avon and is worth a look.

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“There is no more lovely country than Monmouthshire in early spring. Nowhere do the larks sing quite so passionately, as if somehow inspired by the Welsh themselves. There is a blackbird on every thorn and a cock chaffinch, a twink as they call him there, on every bush...... It moved me profoundly. I had been spared to see another spring, and I thank God for it.”

Oliver Kite
“A Spring Day on the Usk”
A Fisherman’s Diary

Don't know how easy Dorchester is to get to but there are day tickets available on the Frome you all so have Richard Slocock's waters on the Piddle.