Sanding reclaimed woodblock floor

After about a year in progess, I am just about ready to sand the floor. Although I've read everything, including WoodYouLike's handy guide, obviously MY floor doesn't conform to anything else exactly , and I need to know if the same recommendation still apply. The details:
* Room is about about 5m by 5m.
* Blocks are reclaimed pine (I think. Probably. Most likely. Not hardwood, anyway.) They are 21cm long x 7cm wide x 4.5cm high. I spent many afternoons scraping the bitumen off the bottoms, so they were 95% clean on laying. They are laid in a herringbone pattern (right angles to wall, not on the diagonal). Around this is a border of larger blocks (the originals from the room that were still usable), leaving 1 cm gap to wall, which will be covered by skirting, yet to be installed.
* Used Rewmar polymer -- expensive, but reasonably easy to use, and no odour. Seems pretty well stuck down.
* Most of the herringbone sits pretty tight, with gaps of 0.5mm to 1mm here and there, and maybe up to 2mm in one or two places where the *&%$£! blocks were slightly off-size, I think.
*Biggest issue is the different heights of the blocks in some places. Tried to ditch the most worn ones, but some slipped through. So there are a lot of differences of around 1mm between adjacent blocks, and up to 2mm in one or two places. Would I be better off digging out the two or three worst blocks and replacing them, or can sanding cope with that big a rise?
*Belt sander or drum sander? Have had one recommendation that it's too big a job for a belt sander, but am aware of the marking problem with the drum.
*Are there different qualities of hired sander? The range of weekly rates at the local hire shops is huge! Anything to ask before taking one?
*Grit recommendation, given these details? Still 40 - 80 -120?

And anything else I need to know?

Not sure why you say a belt sander wouldn't be able to tackle this job? Or do you mean a hand held??

A professional belt sander is THE tool for a job like this, but most normal hire centres don't carry them. A drum sander (specially at grit 40 which you need to get rid of your height differences) will create scatter marks.

Grits: end with 120 if you want to oil your floor, end with grit 150 if you want to varnish/lacquer your floor

Thanks WYL!
Quick search says most hire places only have the hand-held belters, or the big drummers -- is http://www.floorsanding2u.co.uk/sanding_machines.html
the right one to use?

Also -- the main question is how big a height difference between blocks is okay? Will the sander deal with the 2mm differences, or do I need to go over these first and knock them back a bit? Do I need to dig the two or three worst ones back out? Basically, how many mm does the whole sanding process remove?

And, planning to hardwax-oil.

Since you're floor is ;softwood' I think 2mm won't be a problem. If it is, you can always revert to first manual removing the biggest differences locally and then go over it with grit 40 on the Belt sander.

Be aware that natural HardWaxOil on Pine could result in 'orange' colour (when it matures over 1 -2 years). You can avoid this by using a coloured HOW, on Pine and Chestnut we always use and recommend Golden Oak colour.

oil and hardwax oil may turn well turn your blocks orange. If it was me i would use bona mega or traffic lacquer to keep the colour lighter and when you fill the gaps with sawdwust and filler it wont show up as much.

a lot of oils reccomend sanding no higher than 120 grit always follow the instructions on the back.

As you are in the Northampton area there is a hire company who hire "Professional" belt sanding machines and not the lightweights you get at most hire shops.
Google "Hirecentres" and go for the first non sponsored link.
A decent floor sander should deal with a 2mm difference in a pass or three on a 36 or 40 grit.
You should never jump more than 1 grit size on a sander so I would suggest 40 - 60 - 100 and then finish on 120 or 150. (150 grit are harder to find but will give less grain raising if you use a water based lacquer)
Alternatively if you can get a buffer machine, finish on a 120 grit and then "Screen" the floor with a 120 or 150 grit screen.