Windows 8.1 updates failing to download unless I use VPN

[#1]

I have been testing a clean 8.1 install on my spare PC for 2 weeks, I have been unable to download all the windows updates which has obviously meant I have done little testing as thats the first step.
Well tonight out of curiousity I thought I would rule out plusnet by enabling my VPN on my router and BAM the updates downloaded with no issue.
I dont know what to say and think here, plusnet do traffic manage, it seems from where I sit the windows update traffic on my connection is been over meddled with, since I am on a ipv6 test account I will retest with my normal account, but this testing has been done over ipv4 (disabled ipv6 in windows).
By failing to download I mean they will partially download and then stop, they will just pause forever until I intervene, including when I have left it overnight.  As soon as I enabled VPN they downloaded.
I just had a thought I am wondering if somehow mtu is an issue so I will test with a 1492 MTU, if I dont reply back on the MTU it means it still fails.

[#2]

not going to make false info, so I am confirming it looks like using an MTU of 1500 caused this issue so I am leaving my router on 1492 now.
Sadly tho after I recconected I had to do more hopping as pcl-ag07 or my BTw routing to pcl-ag07 appeared congested.

[#3]

Very good spot on the MTU, it's not something I've seen many issues with recently but definitely worth keeping in mind.