Calculating how worth it ditching my fix could be

Daniel7583

I've been playing with the ditch my fix calculator, which says I could be better off switching my 1.99% current mortgage, if I can get a deal of 1.66% or lower. I can, but given the difference isn't dramatic, I wanted to calculate how worthwhile it could be. Would be very grateful if anyone could help check my working and also let me know whether I'm missing a trick - this has given me a serious headache!
I owe 358000 on a 7 year fix at 1.99% - fix ending Dec 2023. ERC is 1%. Monthly repayment is 1434. LTV is 45%.
I don't want to shorten the length of 'fix' that I have, so looking at another fix of at least 4 years.
Best option looks to be 5 year fix with Nationwide at 1.54%. Monthly repayments of 1351.
How 'worth it' is it?
NB. I only have 3 years and 10 months left on current fix, but using 4 years for ease. I am also only considering the first 4 years of the comparative mortgage (seeing the extra fix year as a bonus - does this make sense?) 
As I calculate it, current mortgage costs me 68656 over next 4 years, compared to 64858, so a saving of 3798.
At end the end of the 4 years on current mortgage, balance would be 316562, compared to 313877, so a reduction of 2686.
3798 + 2686 - ERC of 3580 - 125 exit fee - 999 arrangement fee = I would be 1780 better off overall at the end of the comparative period.
Pros
I really liked my previous Nationwide mortgage - in particular the ease of overpayments. 
We gain a bonus year of a low rate fix.
Cons
Locks me into a higher ERC for another fixed period. Not too concerned about this as we won't be selling in that time and I hope we would be able to make overpayments.
I would be really grateful for sense checking my working please. Do I need to consider saved interest from the reduced capital? Are there any extra potential fees I should be factor in?

getmore4less

Add All the fees to switch to the amount borrowed make the payment the same and see what's left at the end of the current fix period.
With over £4k of fees you have to factor the cash flows that's why you add them and make the payments the same.

Paying all the fees makes a smallish difference to the comparison you save less, just treat it as a big overpayment on the current deal before doing the comparison. 

Thrugelmir

In remortgaging you will incur the following (unless the cost is covered by the product). 
Mortgage exit fee, valuation fee and solicitors costs. 

Daniel7583

Thank you both. I've seen rates have even dropped slightly further since.