The fourth expedition to Grove Farm in search of the Unholy Grail of biodiversity - Japanese knotweed - brought together volunteers from the local area, from the far south of the borough, and even from beyond its borders. Mike from Friends of Grove Farm decided to lead the task unusually in the afternoon, due to an earlier event at the Grove Farm Nature Reserve - the Slow Worm Safari. Mike told us that only one specimen had been spotted during the event, while the expected number was ten, which wasn't good news for biodiversity. On a positive note, the sighted slow worm was pregnant, so there's still hope that the local legless lizard family will grow.

The quest to find the highly invasive non-native knotweed attracted adventurers like GoodGymers Giles and Jo, who cycled from the South Ealing and Brentford area, Sevan and Kash, who ran, also from the south, having completed two missions on the way, and Afshin, who returned for the second adventure of this kind in a row. The GoodGymers were joined by a returning explorer, Deborah, and Sue, new to Grove Farm. A team of eight resumed their travel up the Coston's Brook ditch.

The regulars knew the drill, the newbies quickly got up to speed: hedge trimmers, shears, loppers, rakes. Clear the path. Make it wider. Put the trimmings aside. Throw logs away from the trench. Do not hurt the other folks in the process.

Today's discoveries:

• Badger burrows
• A rusty shopping trolley
• A tyre
• A single wellie
• A sandbag-reinforced campsite (a secret bunker entrance?)

But all of those artefacts paled in comparison to what we discovered at the end. Even the knotweed. Oh yes, we eventually did reach the knotweed, although some of us, after three previous sessions, had started to doubt whether we'd find it this year. We left the invasive knotweed alone, acknowledging the mission's success from a distance. Then all the eyes landed on the most unexpected finding, waiting for us under the crown of a large fallen willow whose branches Mike meticulously sawed off.

It was a geocache. Afshin opened the container and found an Elvis CD and logbook with an entry from 23rd May 2005! Everything was soaked, and we couldn't reveal other pages of the notebook without destroying them. The artefact from over two decades ago now needed some drying before getting reinstalled in its location.

Meanwhile, a much younger thing that could be found in the park needed quite the opposite - plenty of water. On the way out of the woods, we visited the black poplar tree the GoodGymers helped plant in December last year and lent Mike a hand in watering it, before returning to the tool container to tidy and lock up the equipment.