You're getting a hard time here OP because what you've described doesn't make any sense.
The rate at which you can pay back the money (which is on top of the elevated standard outgoings you have described such as utilities and mortgage payments) is at odds as to why you would need it in the first place.
You say you got "behind" because of those items being increasingly expensive plus Christmas.
It simply doesn't add up that you can throw £1k at those issues now and then pay £300 per month to repay the loan without falling behind again.
So something doesn't add up.
If your circumstances where that you'd had an unexpected bill of £1000 that needs to be paid now, that would make sense in so far you can afford to pay it, just not in one go- but in that case your first port of call is to see if you could pay that bill in instalments rather than asking your parents for a loan.
However, you specifically mention your mortgage which is NOT an unexpected bill.
The only other thing I can think of is if your wage is variable such as you own your own business and have had a minor cash flow issue but expect a number of payments will come good in the next 3 months (though again I'd question how you are managing your finances as most self employed people keep cash reserves for such a scenario and smooth their earnings over the year).
However when all is said and done, as a parent and if I could afford it, I'd give you the money (never mind lend it) as long I was confident that the circumstances that led to this issue was a one off and beyond your control rather than a result of being financially feckless.