Do I need this washer?  

Re: Do I need this washer?

is that from the top side maybe (the PTFE tube holder)? I don't remember seeing that ever, but ive only been inside mine once.

Re: Do I need this washer?

No there is no washer between these two details.
There are probably remnants of filaments that for some reason are stuck there.
Be careful when mounting your hotend so that filaments cannot leak out from the threaded part.

Re: Do I need this washer?

ah good to know, that may be it then (ie leaked filament) - should there be a small gap between the heatsink and the hotend?

I note if I screw the hotend block in fully to the heatsink, then the nozzle no longer screws in fully as the threads hit the screw in the right of the below picture.

Want to naturally have them tight together, but not sure best way to achieve this!

Re: Do I need this washer?

Want to naturally have them tight together, but not sure best way to achieve this!

Here is the official manual.

Re: Do I need this washer?

super! ta for link, very useful - especially as it's just mentioned thermal compound that you put on the threads.

Any ideas on any stores where I can get this in the UK locally? (ie can't bear another few days waiting for the interwebs to deliver me stuff to get my Prusa printing again...! 😉 )

Re: Do I need this washer?

you can get thermal paste/compound from any computer store or amazon

Re: Do I need this washer?

Ah, great to know, so it's just the same as the standard CPU thermal paste? Ie

https://www.amazon.co.uk/Arctic-Silver-Compound-AS5-3-5G-ArctiClean/dp/B01I74QON2/ref=mp_s_a_1_3?ie=UTF8&qid=1551803386&sr=8-3&pi=AC_SX236_SY340_FMwebp_QL65&keywords=thermal+compound&dpPl=1&dpID=51TYA6bvbIL&ref=plSrch

Re: Do I need this washer?

yes it is and the cpu stuff is higher quality

Re: Do I need this washer?

Ah perfect, in that case I've got tonnes! (Always have a tube left over from every CPU upgrade!)