Apologies if the following is stating the obvious...
Firstly around Europe Spring Tides occur about three days after a full moon. A lot of diaries carry the phases of the moon so that can be used as a reference. Also Spring Tides have a greater range in the Northern Hemisphere winter than the summer (the sun is further away in the summer so there's less gravitational effect). So the good news is that slacks are generally longer in the summer.
Using a suitable Admiralty Chart and the tidal diamonds is a good start (get the one that covers just the area you are interested in rather than just the English Channel) but bear in mind these are always referenced to a specific port (i.e. Dover) so High Water at the diamond will be given in the related table as so many hours before or after High Water at that port. So you also need a set of tables for the port for the period in question (i.e. the 2006 Silk Cut Almanac) and then apply the offset.
An alternative is to buy a set of local tables (dive shop, fishing tackle shop or similar). These are often given in local time (i.e. corrected for BST/GMT) whereas tables like the Silk Cut work in GMT only.
If you don't want to plan too far ahead (i.e. <7 days) then
http://easytide.ukho.gov.uk/easytide...ide/index.aspx
may help with the basic tidal information.
These tools will only give you the time of HW/LW it may not give you the time of SLACK water. There are many places around the British Isles where the slack is before or after HW/LW by a substantial period of time and places such as Lyme Bay are notorious for having two HWs in one 12 hour period. Some wrecks even have their own tables!
For wrecks getting a copy of "Dive...." (i.e. Dive Dorset) will help in determining when Slack water occurs with respect to HW/LW and when the longest slack occurs.
If you really want local diving info' then try and find the local dive club in the area (i.e. start with
www.bsac.org) as they will tend to dive the area you are interested in regularly and may be able to fine tune the "two hours after HW at..." that you can get in the manner described above.