When with the brewery (for 14 years) I could not bring myself to "cheat" the customer, therefore I think that my highest ever surplus was a one off 4.2%, the others were uniformly around the 3.5% to 3.7% mark.
You say Smiffie has told you that most of his pubs are achieving 5%, I think he is telling porkies.
You can't get a surplus on wine or spirits if you are dispensing correctly. You can't get a surplus on nuts and crisps etc.
You can however get a surplus on post mix, cordials, and draught beers and lagers.
Obviously the bigger the head on beers and lagers the bigger the percentage surplus margin.
To boost your surplus percentage you must slam ice into the postmix drinks whenever you can get away with it, you know like MacDonalds do, and don't fill them right to the top, leave a small deficit, you know to avoid spillage.
As for Cordial, you will have a cordial key on your till perhaps ?1.50 for a pint of orange cordial, instead of hitting this key, hit the splash key 5 times assuming its 30p a splash, the money is correct but you know yourself you haven't put 5 splashes of cordial into the pint pot, perhaps you have actually put 4 in giving you 1 shot i.e. 30p hidden profit towards your surplus, again bang some ice in.
Sam Smiths pots are brimfill, i.e. a pint to the top, therefore you can't dispense a pint of lager and lime or lager and black, you have to short measure the draught pint to accomodate the cordial, you must ensure that you charge for the splash thus a shorter draught product measure will increase your surplus.
You are allowed pump cleaning once a week, stretch this to (without fail I might add) every 10 days i.e. 3 times per month instead of four, you will then accrue a weeks pump cleaning allowance for nothing plus you will have sold what you would normally have thrown away at the cleaning process.
I might add to all this, never ever buy in your own stuff to mix or replace a product to enhance your surplus, not even cordials. Remember its not just your job that you will lose, its your home.