Hi ya HamishT I have to agree with Yastiandrie's comments.
I recently migrated from ADSL2 to FTTN and initially opted for the standard 25/5 speed rate and it wasn't until I then saw my modem stats that I opted for the 100/40 speed.
Here are my modem stats for comparison.....................note the maximum line rate.
DSL Status Up
DSL Uptime
16days 2hours 37min 31sec
DSL Type
VDSL2
DSL Mode
Fast
Maximum Line rate
51.61 Mbps 139.42 Mbps
Line Rate
40 Mbps 109 Mbps
Data Transferred
4030.61 MBytes 3553.69 MBytes
Output Power
7.3 dBm -2.2 dBm
Line Attenuation
2.5, 12.3, 18.6,N/A,N/A dB 7.4, 15.7, 22.4 dB
Noise Margin
13.8 dB 14.1 dB
Here's a speed test I've just done.
Do you know your distance from the node?
In my case I'm approx 250 metres from the node.