Hi David,
very worrying. 2mm or 2.5mm wire (galvanised or stainless) should not streach to any noticeable extent. Far more likely is that it is something else. Are the Talurit swaged sleeve splices
http://en.wikipedia.org/wiki/Talurit_swaged_sleeve
in good condition ? If not, replace any shroud or forestay with faulty splices.
You don't say if shrouds and forestay are galvanised steel or stainless steel. Galvanised steel were used on a lot of UK boats & kits in the 60s & 70s & maybe later and will rust in time. If yours are galvanised and starting to rust, I suggest you replace them with stainless steel.
Might be worth checking the lengths, just in case. There should be 2 of equal length (to within a few mm) and one which is shorter (I'm pretty sure it is shorter). The ones of equal length are the shrouds, and go at the sides. The other one is the forestay and goes to the front of the boat. If you have 3 of different lengths, something is wrong !
If the shrouds are equal length and all 3 are in good condition, it must be something else. I'm assuming you have the mast in the aft mast step and are using the aft set of shroud plates, or have it in the forward mast step and are using the forward set of shroud plates (amazing how often boats on e-bay have this wrong and the mast at a crazy angle). I'm also assuming you have the pins in any shroud plate adjusters in their normal positions and these are the same on both sides. If you are using shackles, these need to be the same size on both sides.
As shrouds are 40 years old, you have a gunter mast and gaff, in which case maybe they are not correctly located where they loop over the thinner section right at the top of the mast. It's normal for shrouds and forestay to dig into the mast a bit at this point, but that should not affect the length to any noticeable extent. If one were to slip down onto the thicker section of mast, that would give problems like you describe.
The only other thing I can think of is that there is some problem with the mast step/foredeck so the mast step is not supported properly, or the foredeck near the stowage bulkhead is collapsing. I've never seen this, but I guess it's possible.
Maybe you could look at these possible causes and let me know if any of them help you solve the problem or you need more help.