Option 2 is your cheapest option, but the spark would have to be satisfied the system was safe, and at the very least determine they cable type and condition before deciding if it can be safely reused, and if so what the load limit should be for it. However it might not be sufficient for your goals.
Option 1 with 6mm SWA certainly would meet the safety aspect, the issue is the total load limit you can get away with. To compare with what is already in, and based on 11.5V and 50m cable drop your limits are:
| | Vdrop | 11.5 | V | | |
| | Length | 50 | m | | |
CSA (mm^2) | Dia (mm) | R (mOhm/m) | R (Ohm) | Max on VD | Max 4D4A (70C) | Max I (A) |
1.5 | 11.8 | 31 | 1.55 | 7.4 | 22 | 7.4 |
2.5 | 13.2 | 19 | 0.95 | 12.1 | 29 | 12.1 |
4 | 14 | 12 | 0.6 | 19.2 | 37 | 19.2 |
6 | 14.4 | 7.9 | 0.395 | 29.1 | 46 | 29.1 |
10 | 18 | 4.7 | 0.235 | 48.9 | 60 | 48.9 |
16 | 19 | 2.9 | 0.145 | 79.3 | 78 | 78.0 |
Diameter is for 2 core SWA to BS5467 from Prysmian data sheet. The SWA resistance might also limit the current on breaker trip but you could get round that at modest expense with an up-front 100mA S-type RCD, or look at using 3-core SWA (slightly larger diameter) so you can disconnect in under 5s on a short.
Realistically 6mm is the max that you could expect to pull through at 20mm ID pipe, so 29A is your limit unless you play with adding a joint to 10mm or 16mm, etc.
Option 0 is using split concentric provided you can be satisfied it is sufficiently protected. If all of the buried duct section is under concrete already then it would be OK, otherwise as davesparks points out, it needs protection against damage if in the ground and anywhere it could be dug up in the future. It may be practical to put in duct to BS EN 61386-24 for those areas, but that would need to be factored in to the overall cost and the cable is not cheap, at £6.36/m for 16mm (first on-line figure I found), but is has more usable CSA per diameter than other cables (and is
far better suited to outdoor use than T&E!)