I have 17th or 18th of April as my 1st trip of the season

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You may have more time than expected to sort some rigs out. Here's a quote from the Royal Quays:
"Date for Annual Lock Shutdown (Tuesday 6 to Monday 19 April 2010)
We planning to undertake a closedown of the lock between the above dates. During this time it will not be possible for boats to lock in or out of the marina. The dates are provisional at the moment, but will be confirmed shortly. The shutdown will allow for essential maintenance work and will involve dewatering the lock."