I've asked a couple of questions before and had some really great help so I'm hoping you fine people will be able to help me again!!

Right, grovelling over.. here goes!

I've got a couple of guitars I'm restoring.. one is a Epiphone Les paul and the other is a strat with a fixed bridge.. everything is working (electronically as it should) however I've put two brand new necks on them.. from specifically the SAME MODEL GUITAR from the SAME MANUFACTURER.. however, both guitars once strung up have major fret buzz on nearly all strings between the first and third fret.. rest of the neck plays completely fine..

I've looked online and spent ages looking at videos but can't really find anything that addresses the issue..

Any help would be sooooo appreciated!! Thank you!!
It's hard to tell without seeing it in person, but my 1st guess would be the necks need shimming.
What have you done so far?

Truss rod adjustment, bridge adjustment, nut slotting, fret level?

Neck shim is also a good suspect, but personally I don't like doing it until the other issues have been addressed.
Haha.. so far I have just replaced the neck like for like.. re stringed with a bit of tweaking on the truss rod.. is it likely to be lots of hassle then? I've never done neck shimming..? :/
If all you have done so far is tweak the rod a bit you've got a long way to go before you've done a proper setup. Necks are not drop-in parts, they need some work to be dialed in properly to the guitar they're installed on.

Check the nut, check the bridge, check the frets. Shimming is an option but I think you'll want to get the other stuff checked out first, there's not much point in shimming the neck if the frets aren't level or the nut is cut wrong. You'll just end up with a bad neck angle and the same problems.

Again, plenty of tutorials on a full, proper setup online. Do that first.
I agree with Roc, lots of other things to check before worrying about shimming, if that's done at all.

First thing to do is raise the bridge. Once you get it raised enough the frets near the guitar body stop buzzing, check the nut slot height. If you fret it at the 3rd fret, you should have a very slight gap between first fret and bottom of top or bottom string. Just enough you can see it move of you press the string down in general, if you want to be specific, around .005".

If you get bridge height (action) in decent shape and nut slots seem to be ok and you still have a lot of fret buzz, then you might need to shim the neck. That basically depends on whether you can get the bridge to raise enough, if it will adjust high enough there's no need to shim the neck and give it more angle.

Once both nut slots and bridge height are taken care of, then check neck relief. (truss rod) Capo at 1st fret, fret at the 14th, about where neck joins body. At the 7th fret you should have about .010" to .015" between fret and string. For more neck relief, loosen the truss rod, for less tighten it. No more than 1/4 turn at a time, then let is settle in overnight. The wood does not snap into place immediately, it takes a while for any adjustment to take full effect, so let it alone overnight before trying any more adjustments.

Then set intonation...with new strings...
Hmmm...I wonder what this button does...