Relocked HTC One stuck in boot loop and won't enter Recovery

A

AC Question

I rooted my phone (HTC One, bought in the UK, not branded) a few months ago, though I never installed a custom ROM or anything. I recently (3-4 months ago) re-locked it to receive OTA updates, though that never worked, but thats another story.
Now I havent had any problems since, except today it was just sitting on a table and started rebooting itself. The OS does load, but after about 10 seconds it just reboots.

Now I read that clearing the cache might fix whatever is going wrong, but I can't access recovery. When I enter HBOOT, it says both Tampered and Relocked, fwiw.
I can select Recovery, and I get the HTC logo saying "entering Recovery", but after a split second it's gone and I'm back to HBOOT. I did make sure fast boot was not selected in the phones settings.

Can anyone help me out? I have no idea why it just decided to do this, as I havent installed anything or changed any settings for ages, and Im travelling without a laptop so I cannot use fastboot cmd prompts from a PC.

Thank you for any help!!