I definitely would recommend gosfield lakes, they practise mass starts on tuesday nights, the organisers are really friendly and anyone is welcome, last year it cost £2 each session with no commitment to any particular number of sessions, just pay on the day.
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Open Water Swim Sessions (From Sunday 27th April 2008)
Gosfield Lake Resort, Church Road, Gosfield, Essex, CO9 1UD
Sunday Mornings 630am - 845am
Tuesday Evenings 630pm - 9pm
The open water swimming is open to any triathletes / open water swimmers, it is not just for Born 2 Tri members.
Hope it helps
see you there