When looking to buy a secondhand Merlin in wood what builders are best recommended and which (not wishing to upset anyone) are to be avoided.  I appreciate it depends some what on the condition of each boat but perhaps someone can point me in the right direction.

Early builders 50-60s, Adur,Hoare,Holt(Jack),Chippendale, Anderson Rigden and Perkins
70s to 90s Smart, Rowsell, Aln, Guy Winder, Turner also Anderson Rigden and Perkins(ARP)
90s onwards Turner, Rowsell, Smart, Chipstow (Alan Jackson), Winder, Full Force, Kevin Driver.
Most modern boats are now 'Plastic' not many wood ones being built
Many builders had a go and some are good other not so, some were well built but tended to err on the heavy side.
Even the good builders in the 70s, as the boats got wide, could have an issue with glue powdering (especially after 30 years) until epoxy was introduced

It all depends on what you are after

You can add Freeman of Shoreham to the 1970's builders, I had 2789 which was 1973 vintage and the only dodgy areas were where neglectful previous owners had allowed fresh water to accumulate. If contemplating buying an old wooden boat pay particular attention to the inside of the forefoot and buoyancy tank(s), the inside planking to hog joint and the inside bottom of the transom. The other main area where joints may have been strained by hard sailing is the centreboard case but the builders already named in this thread made sure that it was adequately re-inforced in their boats. 
Good luck, Peter 3112

You can more or less be assured that the builders that were well established in their day built a quality boat. What tends to drive people towards recommending a particular builder are aesthetics and the record of the boats they built, which in the 60's meant mostly Bob Hoare, Jack Chippendale and Wyche & Coppock (The builders licensed to produce proctor designs), the 70's and early 80's were dominated by Rowsell's and in the 90's a very limited pool of boats by Jon Turner were highly sought after.

This has lead to some builders being very underrated, most notably Aln Boatyard who produced excellent and probably the strongest boats in the fleet during the 70's but lost out mainly due to - of all things - their location, up north and the fact they did not go in for the "finery" of laminated carlins and fretwork knees.

Even the best built boats have their weak points, for example Rowsell and Turner built boats have very pretty laminated carlins but these are prone to splitting especially if the boat is or has been neglected. In this respect you need to carefully check any potential purchase for signs of weakness. Don't be put off by repairs unless they look ropey, but always check the opposite side of the boat for a similar problem especially with failed or repaired glue joins.

I can confirm the credentials of the Aln Boatyard for building strong boats with great longevity.  The Entertainer 2706 dates from c.1973 and is still sound.  Minor structural problems have been easily fixed with handi-pack epoxy and the appropriate filler.  Currently languishing under a cover in the gaden awaiting better weather for a comprehensive epoxy and varnish to put good the ravages of 12 months student sailing and outdoor storage.