Advice regarding partial garage conversion

Doozergirl

What i found is, this is a project which seems small for many builders. So i am trying to approach bathroom fitters with decent reviews and recommendations for quotes. I have few questions for advice as  below ( as I am a total noob ) 

I think this is beyond many bathroom fitters.  Building control are involved here and they rarely would be in a bathroom refurb.  

1) what would be a ball park figure if i provide , both the doors, shower enclosure unit, WC, vanity unit , tower rail, tiles for wall and floor.
I can't answer this one as
a) the drawings aren't to scale enough and I'm not quite sure if/why you're doing half the floor.  the scope isn't that clear to me, which is why some builders may not be interested. 
b) I have always provided materials and labor and therefore a full warranty

2) Will i be able to get the above items ordered by the fitter ( so that I can save on VAT ) 
Nobody saves on VAT unless you're building a brand new house. 

3) How  many days does this kind of project take.  
Maybe 2-3 weeks

4) Will I be able to request a broken down quote. 

Will depend on your builder but if they do it will be broken according to the way they price rather than the way you expect it.  

5) Is it worth to source multiple tradespeople and manage them  or should i go with some one who can source and manage.  what would be the % increase in cost in the second case? 

This isn't a % increase.  It's like trying to build a car by buying the parts and have different people put them together for you.  If you and they combined have the right skills, you'll save quite a bit of money.  If you don', you'll make mistakes along the way, you'll cause extra work and that margin dwindles away.   You may well save money but you may also beed to do it again.  It's your risk.    

6) Will there be guarantee of the work normally provided ? 

Your statutory rights give you 6 years if you buy through one contract.  

A builder who is Trustmark registered should offer you a 10 year insurance backed warranty as part of their contract and members of the Federation of Master Builders have that option too.  

If you buy your own materials and employ separate trades then you don't have what is now being called a 'golden thread' of responsibility.  I made a living from that, so it's perfectly feasible but you can't have the moon on stick - either they are responsible for everything or you are.  

Thanks .