Hi Peewee,
As said by Pete bracing is essential! Of course the ply backing will provide this too.... As long as you use plenty of screws, from the way you have the legs the ply would need to go between the uprights and screw directly onto the horizontals IMHO. maybe put bracing on the back of that if you still think it needs it?
A very useful thing to be able to do if possible is fix it to the wall at a few points, if you look at the brackets holding the tracks up on the rear of your garage door (Hormann retracting if I'm not mistaken? I know, how sad am I.... Fit 'em for a living!) they are great for fixing shelves and benches to the wall and floor... They hold mine solidly (no shakes when hacksawing 50mm metal bars!) also pin my mill bench to the floor
What are you putting into the area under the bench? Cause there are some ideas on my thread in this section of the forum.
Pretty cheap tool boxes from Chester UK and the project trays idea I borrowed from another party (named in the post IIRC?) is an awesome addition IMHO
Looks pretty sturdy so far
Where about are you in the UK? Many of us have been/just started adding our general (not exact for security) area on our side bar/signature line (Profile edit) helps us know if we can aid in just text or come visit and talk about tons of stuff

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Hope some of that is helpful?
Ralph.