Summary: don’t bother.
I’ve just got back from dining at The Peppercorn. It’s just been refurbished, so I thought it worth a go. The cider there is decently priced - £2.45 for a pint - but the interior decor is entirely unremarkable. I didn’t notice anything of interest. The menu was short and expensive, with few of the usual bar staples. I ordered the lamb chops - £12.95 - and opted to eat outside since it’s such a lovely evening. After sufficient time for two long phone calls, the food arrived: three modest lamb chops with a roundel of potato in the middle and a scattering of beans, spring onions, and peas. It wasn’t hot. But it was moderately tasty.
Eating outside was a mistake: there was insufficient insulation against the road (granted I was sat next to the hedge, but I’d earlier sat further away with no difference). This is unlike the Old Bell, a few hundred yards up the road where you can sit outside and not be bothered. And the view from the Old Bell is great too.
The price of the food did not match the quality, nor the quality of the place. If you want to eat well in Harpenden, go to the Old Bell. you’ll eat better in a better place for less. Likewise the Wyvern in Luton, which is in staggering distance for me.