Having done a bit of brief research, I find that "legit" is a synonym for "good". So, no. The header announcing a "principal amount" serves no purpose and could be misinterpreted as indicating that there is a loan. There is no reason to indicate the date twice, which gives rise to two different values of "date". The phrase "For value received" can be interpreted in at least two ways, one being "in exchange for some unspecified value to be received at some future date by Promisor", and "in exchange for a specific value already received by Promisor". Under the later interpretation, Promis(s)ee probably could not breach but under the former, Promis(s)ee could. So it makes a difference. You can just pay to the Promis(s)ee, and not imply that you are creating a pay-to-order instrument such as a check. That still leaves you the option to pay with a check. Or was the intent to say "pay on demand"?
The expression "the sum of £500 shall the Promisor fail to meet his target" is not grammatical in US English, and I'll leave it to a UK speaker to judge if this is, over there. I assume that this is supposed to express conditionality, in which case "if" is a useful term. Then the meat of the contract, I guess, is that if the Promisor fails to meet somebody's target of 47.5 work hours per week, then Promisor has to pay Promissee £500 (and not otherwise). It's really not clear how anyone would know whether "meet a target of 47.5 work hours per week" has come to pass. Does that mean "work at least 47.5 hours per week"? Does that mean "for each of the three weeks within the time period" (or did you mean "work 47.5 hours within some one week, within the 3 week period"). Being explicit that the work obligation extends for 3 weeks would be legit (vide supra).
Supposing that the second clause means "Promisor will pay £500 by 5:00pm 2 July 2016", you should put it that way. Or if you mean "Promisor will pay £500 by 5:00pm 3 July 2016", say that. Deadlines for performance should be stated very directly and clearly, and require no calculation and interpretation. And I'd suggest including a clause stating that "Promissor" and "Promisor" are used interchangeably in this contract. Or else be consistent in spelling.
Note that almost any contract can be given some interpretation. From the perspective of creating a contract, the first concern should be over clearly expressing the intentions of the two parties in written form. After all, you don't have a contract if there is no meeting of the minds.