Sarah, sorry for delayed response been away past couple of weeks..
Firstly, I agree with LoanInsider "don't stress" too much as you're in a much healthier position than many. Still good to keep your finger on the pulse though - so well done you..
You're in this for the long term and have only just started. This dip will be tough over the next 3 or 4 years but you'll be ok in 20 years time I'm sure. Consider how much it would cost to rent your home - a 3 bed semi value £130-£140k gotta be at least £600 per month probably closer to £700 depending on where you live. So you'd be throwing that kind of money away each month if renting, thats' why I'm suggesting pay interest only while you're on a great rate (so long as you put that capital equivalent somewhere and don't blow it).
Sorry if the maths confused you. To calculate interest only from the figures you gave, I took the full borrowed amount and multiplied by interest rate. 128250 x 0.0454 = 5822.55 per year interest (divide by 12 to get £485.21 per month interest).
You said your payments were approx £820pm so 820 - 485 = 335 capital (give or take a fiver if I rounded up).
I believe too many people cripple themselves paying off big chunks of capital which to me is senseless in times of such low interest rates PROVIDED you have a better home or action plan for the equivalent money. In summary if you're good at looking after your money (as you would seem to be) keep it in YOUR pocket as long as you can!
If anything alse isn't clear just update this thread and I'll be quicker to respond. Good Luck RS
ps - I stress what I've said in this thread is my own opinion and should not be read as any means of financial advice as I am not qualified to give such advice.