Winter MTB tyres for the Chilterns

I'm after some tyre advice, with quite a narrow brief :)

My two teenagers are riding a lot of MTB at the moment, on their XC race bikes. We are North of Reading, and they head out Wallingford/Goring/Ridgewat direction. The trails are getting really muddy and their XC race tyres are struggling for grip (obviously). I'm also getting frustrated by the amount of tyres we are getting through with rips from the flint around here. 2 tyres in the bin in 3 weeks. Sigh...

So I'm after recommendations for 29" tyres that are tough, can handle mud, but won't break the bank either.

For info, they run a mix of Bontrager XR2 and Maxxis Ikon as race tyres. XR2's have been great, and I can get them discounted, but they do rip easily.
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