Ian,
1) I have used three brands of the Reciprocating Saws (all the cheap end, £50 to £140) and they will all call cut through any thickness if the blades are good and long enough.But it is slower than a chain saw.
2) I have had no issue going through 40cm green trunks (noting that I tended to take top off tree first and then drop lower trunk section section last).
3) I always use the Bosch blades.
4) The issue is that the longest blade I have used/seen is 24cm, so it takes longer than using a chain saw as you have to consider and do the multiple cuts (and this then gets more dangerous as 40cm trees are dangerously heavy).
5) But I have felt safer with a Rec Saw than a chain saw when up a tree.
6) But Rec saws are much slower than a Chain Saw.
So in conclusion,
if you have lots of 40cm trees, get a chain saw, checking in advance that new chains are easily available.
If a one off or occasional job, I personally like the Rec saw as I can use it all the time on other projects (DIY, demolition, medium tree pruning etc).
Looking at
Screwfix, do not get the cheapest electric chain saw as too many give it one star (I did consider this saw). Instead look at the £140? Makita chainsaw.
And if you do get a Rec saw, throw away the blades it comes with and buy the good ones (I again recommend the Bosch ones).
SFK