Cabling from OR modem to Router

[#1]

Hello all, here's a fairly random question about fibre broadband...
I was hoping to have my router nearer to my PC so when signing up I'd requested a data extension.  When the engineers turned up they said "you don't want that - it'll slow your connection down"  Fair enough, I didn't want masses of cable running through the house so I settled for running the modem through about 10m of cat 5e into the downstairs cupboard (which I'd previously laid myself)
The engineer seemed surprised that the router synced with that length of cable...
Have I completely misunderstood how this works? I thought the OR modem synced with Plusnet and you could run as much ethernet to your router as you fancied? Does the cable to the router make a difference?
As a second (first) question - does rebooting the router (not the modem) mess with the DLM?
Thanks for any advice/help on this.

[#2]

Using the CAT5e from the master socket to the modem increases the distance from the cabinet and can slow things down a little.
Using CAT5e from the modem to the router is essentially a normal network connection and you can go up to around 100m without a problem (around 60-70m with coated aluminum CAT5e)
The odd reboot of the modem will not cause a significant drop in speed, neither will a reboot of the router although it is usually good practice to log in and manually disconnect the connection first.

[#3]

First thing
I bet the engineer was from Kelly as he was talking total rubbish
A proper extension will have a very minor effect on the speed as the length you would be talking about would be a very small proportion of the length to the cab
The sync is done at the modem - the cable from the modem to the router can be any length within reason (without checking - up to 100m with that cable spec)
Second question
Rebooting the router has no affect on the DLM as that system only looks at the sync to the cab not whether you have a valid PPP session
Edit Dom types faster than me 

[#4]

Wow, thanks for the quick response. I've got a short cable from master socket to modem. The engineers were from Kelly. lol. Nice enough guys though. Guess they couldn't be bothered routing longer cable. Good to know I can move the modem without worry and get 8 flashing lights out of my 4yr old sons bedroom! Although tape works wonders (on the router, not the son)
Good news about rebooting the router too - I'm paranoid about doing it as I've suffered achingly slow <2mb broadband for 4 years and want to get the fastest fibre connection I can.
Have a great evening/day and thanks again.

[#5]

I installed a CAT5e socket next to my VDSL socket, ran it to another CAT5e socket where I wanted my modem and router to reside then used a patch cable between the VDSL socket and CAT5e socket then just plugged the modem back into the CAT5e socket at the other end.
I noticed a small reduction in speed but nothing major and it removed the modem (and its flashing lights)  from the bedroom.