Thanks for that, it's along the right track of what i wanted (though like you say the maths is slightly off). I've tried to fix the maths of it, but i keep breaking it somehow 
Also, i've missed out a field from the form by mistake so the form should be
<body>
<strong>Gross Profit Calculator</strong> <br /> <br />
<label> Case price (exc VAT):
<input id="CasePrice" data-bind="value: CasePrice" value="100"/> </label> <br />
<label> Units per case :
<input id="UnitsCase" data-bind="value: UnitsCase" value="12"/> </label> <br />
-------------------------------------------------------------
<br />
<label> What % GP do you want?:
<input id="percGP" data-bind="value: percGP" value="0.10" /> </label> <br />
<label> Cost price per unit:
<input id="costPrice" data-bind="value: costPrice" value="" readonly /> </label><br />
------------------------------------------------------------- <br />
<label>Selling Price Per Case (exc VAT):
<input id="sellPriceCase" data-bind="value: sellPriceCase" value="" readonly /></label><br />
<label> Selling price per unit (exc VAT):
<input id="sellPricexVAT" data-bind="value: sellPricexVAT" value="" readonly /> </label> <br />
<label> Selling price per unit (inc VAT):
<input id="sellPriceVAT" data-bind="value: sellPriceVAT" value="" readonly /> </label> <br />
<br />
<label> Cash margin per unit:
<input id="cashMargin" data-bind="value: cashMargin" value="" readonly /> </label> <br />
<br />
<button onclick="calcGProfit()"> Calculate GP </button>
To help with the maths it should be worked out like this
Cost Price Per unit (ex VAT) - case price/units per case
Selling price per case (ex VAT) - cost price per unit/gross profit * units per case
Selling price per unit (exc VAT) - Selling price per case/units per case
Selling Price Per Unit (inc VAT) - selling price per unit (exc VAT) * 20%
Cash Margin Per Unit - Selling price per unit (minus) cost price per unit.
I realise im probably pushing my look, and i'll keep trying it myself, but if you could help with this bit that would be awesome ! 
Thanks for your time!