Adventures Travel Insurance (also called just "Adventures") is a UK-based insurer that specializes in providing travel insurance for adventure seekers and thrill-seekers. Adventures is arranged by P J Hayman & Company Limited.
P J Hayman operates a number of different insurance products, with different names and websites:

Longstay and Backpacker – Ideal for extended trips abroad, on a ‘Gap Year’ or round the world trip.
Euro Plus – Motor Breakdown Cover for people travelling to Europe by car.
Free Spirit – A specialist travel insurance policy for people of any age with existing medical conditions.
Cancellationplan – Specially designed to cover holidays in the UK.
Travel Plus – Travel insurance with ‘added value’ benefits.

Reviews

Check out reviews of Adventures Insurance on Trustpilot .
There are a limited number of reviews for Adventures specifically, however, you can check the reviews for other products that P J Hayman operates such Free Spirit Travel Insurance.

Activites

Adventures Travel Insurance can cover a wide-range of activites. You can search for them here: Adventures Direct activites. However, for more extreme activies (such as trekking over 6000m, mountaineering over 4,500m, etc.), you must contact them directly on 02392 419 093 for a "referral".

Trekking

Adventures Travel Insurance can cover Trekking 4000m to 6000m without referral.

Mountaineering

Adventures Travel Insurance can cover Mountaineering up to 4500m only without referral.

Climbing

Adventures Travel Insurance can cover Rock Climbing (with ropes & or guides or bolted) up to 4,000m only without referral.

Skiing

Adventures Travel Insurance can cover Skiing off-piste (not against local advice).

Paragliding

Adventures Travel Insurance can cover Paragliding in Europe only. They can cover Paramtoring worldwide.

Flexibility

Adventures Travel Insurance offers 3 levels of Policy cover, with a limited "Part A" policy that could be used as an add-on to other travel insurance as well as "Part B" and "Part C" that include further coverage for an increased Premium.
Adventures Travel Insurance does not offer an option for "already-travelling" insurance.

All information provided is indicative and may be out of date - you should always check with the insurer that they will cover your activity.