I've recently been getting frustrated with my stamina when playing , normally just powerchord stuff say Nirvana stuff or you know something powerchord based so kinda rhythm I guess , anyway that's beside the point. But I've noticed after about 25-30 minutes my fretting hand starts to ache and kinda tense up? Does anyone know why this might be or how to solve it? I was thinking maybe hand exercises an just playing more and battling through it but I don't want to risk making it worse!

Thanks , Simon
Sounds like your pushing too hard with your fingers and thumb. Just relax everything. Learn not to push too hard with your fingers. Fret as usual the very gently release the pressure until the string buzzes. Then apply a little pressure until you get a clear note. Thats the sweet spot
Don't apply any more pressure than you need to. The harder you press down, the more out of tune the notes will be. Just press down hard enough that the string makes a solid connection to the fretwire and no more.
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I also have to do that. Cottaging this weekend
Make sure you're fretting with your fingertips whenever possible, not the pads. Ideally the string is just a couple millimeters below the fingernail.
You may already know this, but whenever you are playing ALWAYS make sure that you are fully relaxed. You shouldn't be allowed to feel any pain or tension whatsoever (except maybe for strings cutting in your fingers, but that doesn't seem to be the issue here). This rule can be applied to pretty much every technique; fretting, picking, fingerpicking, hammering on, pulling off, etcetera.. I cannot stress this enough. Whenever you feel tensed (even if it's just a tiny bit!) stop playing immediately.
It does sound like you may be squeezing/pushing too hard on the neck. You don't have to press down hard in order to get the notes to ring clear. Yes, some chords may feel like they require more effort such as the typical F chord shape that beginners tend to struggle with. Hopefully your strings are set to a proper height, so the instrument isn't adding to the problem.

Do you experience this pain with just barre chords, or does this happen when you practice tunes with open chords as well? Practice holding a chord and picking each note one at a time. If you hear each note cleanly, let up the pressure a bit and do it again. See if you can get all notes to ring with as little pressure as possible.

As MyLastWords said, don't push through it. If your hand hurts, take a break.