Taking a Hybrid and use for MTB

Hard tail will be fine if a little bumpy over rough stuff. Jumps will be harder. A decent MTB is much nicer to ride and can save you from crashing on occasions. Tyres, geometry and suspension play a big part in how the bike handles when the terrain get rough. I built a Scott FS with 36mm forks and it ride over rough terrain very well, far better than my ability

1)

You might get away with it on a Forrestry Commission blue level trail at pootling speed, but as comments above have said, avoid losing contact with ground where possible.

There are numerous videos of mtb riders taking somewhat inappropriate bikes down very gnarly trails, but they have the skills to deal with it if the riding gets a little 'unexpected'. A hybrid might get you round simple trails, but the bike will not inspire much confidence over anything more than a rough surface. It may be worth a try for the once, but you can pick up an entry-level 'proper' mtb for £500 second hand (ideally something that way worth £750+ brand new for a reliable build quality on the trails) and that should give you a much more enjoyable experience.

2)

Tyres are going to be difficult. For off road, generally speaking bigger is better and you are limited with a hybrid frame.

3)

The Calibre Bossnut is the cheapest full-sus that's worth looking at, but there's a lot to be said for trying to find a decent spec hardtail on the second hand market for people getting into the muddy side of riding. Whyte have a range of very capable trail-oriented hardtail bikes, which are a blast on the trails and no slouches on the road either, but the best bet is to try around. Look at demo days at your nearest FC centre or speak to your LBS, even if you go down the 2nd hand route, get a bike you like the feel of.

Basic checklist of what to look out for on a decent 1st mtb

- Hydro discs w/ 180mm front rotor (Tektro work, but SRAM or Simano are better)

- Forks; 120mm travel and if your price point allows, air sprung at entry level. (Very few mid-range forks have coils, only bargain basement or very top end)

-1x drivetrain w/ narrow-wide chainring and clutched mech ; May cause arguments on the tarmac, but is a no brainer off road. Mech and chainring will mean you should never drop a chain

-If you can get a bike with a dropper post on 2nd hand, you are on to a winner. Dropper posts are a monumental revalation in riding, one akin to the introduction of bar-mounted shifters.

Bike marketed as "trail" will be more forgiving that XC, which may have twitchier, more responsive handing that might feel more similar to a roadie on the flat, but as bumps and roots and drops start to appear your, face will thank you.

The other thing I'd say, I know plastic hats are a contentious issue on the road, but they are essential on trails.

Oh, and one more thing; enjoy the ride.

The reality is that your bike is as good as any 15-year old MTBs were and possibly a lot better given it has hydraulic brakes which my  MTB bike at the time didn't.

Yes the latest MTB bikes have more suspension, larger tyre clearance and slacker angles, but your bike is perfectly capable* of getting round cross country and even intermediate MTB trails (as long as you are). Just don't go leaping off logs/rocks, etc

*The main problem you have is tyres and that you have already identified. With only 45mm space to play with you are going to find your self looking at the gravel bike tyre selections or cyclo-cross (resticted to 33mm wide for UCI competitions). For muddy conditions, wider is always better.

Consider WTB Cross Boss (35mm), WTB Nano (40mm), Continental Mountain King (32mm).

If the wheels never leave the ground and you stick to well established flat forest tracks and bridleways itll be ok. As long as you fit a grippier tyre, some of the cycloross tyres out there are worth looking at. MTB 29er tyres will fit circumference wise, but might be too wide for the frame, and will happily saw through your frame if anywhere near close to the frame tubes.

Add any kind of difficulty into the riding and it will suffer, and not be up to the job. 

It’ll be perfectly fine I’m sure, but if you want to spend a grand on the cycle to work scheme get a Calibre Bossnut from Go Outdoors. 

Well - only you know what kind of riding you want to do on the bike. Sure you can take it off road but there are some things about it that limit its capabilities.

You refer to "rocks and roots" - the geometry of the bike (head tube angle and standover particularly), the wheels (width and strength) and the fork are the key limiting factors once you start to tackle genuine MTB trails.

Basically, in my opinion it's not suitable for genuine MTB trails but would be fine for brideways and what have you. If you want to ride proper trails, don't bother mucking about with the kit on the Crosstrail - get something more appropriate.

Check out the 2017 Hardtail of the Year from MBR magazine - videos on YouTube. See what you think of their recommendations.

I'm not an expert here but you see so many people riding trails on Cyclocross and Gravel bikes (ie road bikes for trails) these days that you shouldn't have a problem.  I really enjoy cross-country riding on my MTB, as opposed to more technical riding, which I could probably easily achieve on an XC or gravel bike.