Darren
Apologises for taking a while to get back to you, Family and work and a bad back have priority. Please see the calculations below, Note that due to me not knowing the site I have made some broad assumptions with regard to the electrical characteristics at your property!!
As you have stated that you are feed from an overhead line system (TT system for the electrically minded among you)therefore must assume that the Prospective Fault current at you meter tails is going to be around 600-700Amps – best case and the Earth loop impedance will be in the region of 21 ohms
I do not know what protective devices are fitted upstream of the SWA cable (Steel wire armoured) feeding the garage (Man kennel) so I will have to ignore them for the purpose of the following calculations – finger in the wind guesses??? – ( lots of shouts of sack him at this point from all the PhD’s in electrical engineering on this site)
Calculations 1/ - (Quick check total volts drop is less than 11.5volts (5% of supply))
Assume that 20amps load on system.
1/a/ Ring main is 18mV/Amp/Meter for 2.5mm² T&E (Twin and Earth Cable)
Therefore Vdrop in ring = (18mV x 20A x 30M)/4
= 2.7Volts1/b/ SWA Cable is 12mV/Amp/Meter for 4mm² SWA
Therefore Vdrop in SWA = (12mV x 20A x 30M)/2
= 5.4VoltsTherefore total volts drop at around 20A load is
8.1 Volts so even if load was actually at 25A due to lights etc in garage volts drop would be 9.75Volts and at 30A load it would be 10.8Volts . which is still (just) under the 11.5V limit.
2/ -
Calculation of fault currents and MCB operating times2/a/ Assume Impedance of the supply is 0.4ohms.
Impedance of the 4mm² SWA cable is 4.61ohms/1000M
Therefore Impedance for 2 runs of 4mm² SWA in parallel,
Length = 45M Zswa= 0.207ohms
Impedance of the 2.5mm² T&E cable is 19.51ohms/1000M
Therefore Impedance for 2.5mm² T&E cable in a ring main,
Length = 30M Zring = (19.51mohms/M x 30M)/4
=0.146ohms
Therefore total Impedance Zt = 0.4 + 0.207 + 0.146
= 0.753ohmsBut you must apply temperature correction factor of 1.2
Therefore Zt = 0.753 x 1.2
= 0.9036ohms2/b/The Fault current at the furthest point of the ring main should be
Ifault = 238V (measured)/0.9036ohms
= 265AFrom the Wiring Regs (BS7671) the current required to cause a BSEN 60898 Type B 32A MCB to operate in less than 0.4Secs (Requirement of Reg 411.3.2 – portable equipment) is 160A Therefore as the calculated (Caveats noted) fault current is greater than this by 10% (my safety factor), the system as it is complies with BS 7671.
BUT if you replace the existing Type B 32A MCB with a Type C 32A MCB you will not comply with the wiring regs as the Type C 32A MCB requires a fault current of greater than 320A to operate in less than 0.4Secs.!
SOLUTIONTherefore I suggest and it is only a suggestion that you wire a short piece (3meters) of minimum size 6mm² T&E to a 32A commando socket with a suitable isolator and fit a Type C 25A MCB in a separate way in the garage Consumer unit!
This would give a temperature corrected impedance of 0.756ohms
Therefore fault current would be - If = 238V (measured)/0.756ohms
= 314AThis fault current is greater than the current required to operate the Type C 25A MCB, which is 250A ,again this would then comply with BS 7671.
CaveatPlease be aware that the above calculations are based on the limited information I have been given, a site survey might conclude that the exist installed system does not even meet current regulations!!!! That is not a critisim of your wiring skills Darren, just that you have quite long cable runs on a overhead TT system in a rural setting , to quote you “I do still get a bit of volt drop at times, it's not constant but the lathe lets me know when. That's in 2HP mode and cold, it's fine when it's warmed up a bit .... usually.”
For example when the lathe is starting as above there is probably 100A inrush for a few cycles of the mains , given a peak volts drop of approx 30V which is going make the lights dip a bit!!
Anyway I hope the above is of some help – If any of the boffins on this forum wish to comment / tear my thoughts and guess work to pieces feel free

PS I spent ages typing this lot above up in a word document, nice spacings etc and its all c"&*ed up when I copied it onto the forum, arrh well I've tried to correct it, I hope you can understand it.
Best Regards
Fred