My DT360a revs quite easily to 6,000 but then hits a brick wall regardless of the gear.
Will run to 7,000 in neutral.
Running standard jets, main being a 180.
Standard gearing
Has Rex ignition system 
Compression is about 110psi using the kick starter.
Any suggestions please?

Try it without the airbox lid.

While redline is at 7,000, peak power is at 6,000 and peak torque at 5,500.  Power drops like a rock after 6,000 rpm.  It is NOT a high revving engine by any means and I've never ridden a stock DT360 that can pull to an honest 7,000 in the higher gears.

Cheap little electronic tachs are readily available so first thing I'd do is verify the stock tach clock isn't reading low...  I've had several that do read low...  I suspect that after 50 years the lubrication isn't what it used to be and the extra resistance result in low readings.

My next suspicion would be the Rex ignition....  but I'm not a fan after seeing some of the legitimate issues they've had over the years and their refusal to stand behind the problem for some customers.  Hopefully they've sorted out the issues by now?  But even if they have, using the source coils to trigger spark instead of a separate pick up coil might be fine on lawn mowing equipment but no vehicle manufacturer I know of uses that design for a motorcycle CDI...  even the dirt cheap Chinese engines spend the extra cost for a dedicated spark timing trigger coil. 

Strobe it to see exactly what it does at idle & all the way up by making a very visible mark at 2.9btdc from flywheel across to something stationary. Can make your mark anywhere that's easy to view with the strobe but first set piston at the magic 2.9mm btdc if that's what i think its meant to be on all the 360's from day one. 80x70 bore & stroke. ps, i just use white twink correction fluid, shows up good with my strobe.
Not that i'm thinking that could be your issue--if there's an issue--but be nice to know it's doing what it should so far as timing goes.

How does the plug look?

I have had the same experience as Mark. If I'm giving it the beans I shift at 6k or just past it. The stock pipe effectively inhibits high revs. Same pipe they used on the 250. It definitely chokes up the 360.

Agree with the other posters, especially like the note about making a mark at 2.9mm BTDC and then checking with a timing light to see if the plug firing jumps around. After some issues with the CDI unit on the bike I own, I moved to a flywheel mag setup from a '75 DT250. (At that time Rex and Vape systems were not out yet) So far I have not had issues but I do not ride it hard at all. It also has a 'sleeper' pipe and that possibly helps. I would lastly add that before I had the pipe modified I removed the spark arrester for a jaunt around the yard. That really livened things up, motor seemed to rev easily. I believe the OEM spark arrestor is probably fairly restrictive (?).

Yamaha points magneto from a 74 on 250 is a great choice to replace the CDI even today, if you can find one.  Reliable as a rock and it's an "Energy Transfer" magneto design which is sometimes called a "mechanical CDI". 

(Some early Yamaha's (early 1960's) had a standard magneto where the ignition coil was actually mounted on the stator and triggered directly by the spinning magnets...  Briggs and Stratton also used the standard magneto with the ignition coil powered directly by the magnets.  Energy Transfer design Yamaha went to gives a much better spark.)