My model is second-hand from Amazon in 2009 (until I bought it I didn't realise they had such a market). Humax Duovision PVR-9200T to give the full name.
It's an IDE machine. It came with a 150GB IDE drive. Like a man I wanted bigger.
I looked on a UK forum ( http://forums.digitalspy.co.uk/showthread.php?t=1386584
) which seems to have folded. The advice their was that a special AV drive was needed (which I searched around for).
I bought a Seagate DB35.3 750GB 7200RPM SATA 3Gb 8MB Cache - OEM 3YR for £51.31 from overclock in late 2009.
If you have a modern machine it's probably SATA. My main problem was finding an IDE SATA convertor (less than a £5 from HK I think).
My model is well built. Five screws and the lid is off. The hard drive is secured by four screws on anti-vibration mountings. Unscrew replace. Put IDE-SATA convertor in to replaced drive.
It's a trivial job. Even in the reach of men!
If your Humax is a different model I can't help other than say look online or quote the model number.