Supply is TN-S, as stated.
Cable is hessian covered PILC, Paper Insulated Lead Covered.
- The paper is saturated in oil which does seep out, perfectly normal
- Hessian may be quite tarry to aid its insertion through your duct
Note PILC is not "armoured" in the conventional sense.
- Underground you will find the cable with bare lead exposed
- A spade will happily cut & short PN before your BS1361/BS88 fuse
- Assume PSSC outside london is 16,000A, inside london 24,000A
PILC requires care.
- Lead cubic crystalises over time making the cable fragile to splitting
- Never move a PILC supply cable, it is not your cable anyway
Weeping like that photographed is quite normal, indeed you are quite lucky the area is not sodden with oil/tar as many old properties. The only time a DNO is interested is a) if the cable or cutout are warm b) your cutout is metal (or metal & double fused) c) a large quantity of oil is found. Otherwise PILC is replaced a) when it goes bang or b) when you pay for it.
Generally PILC fails at the terminations, typically old pitch filled metal cutouts. Those should have long been replaced - if not then do contact the DNO. DNO will require MEB upgrading if they come out and do meter-change, supply cable change, incidentally. Typically 25mm tails & 10mm MEB (16mm if cable run outside re sized for mechanical protection and some even choose 25mm outside because it covers you if the insulation is damaged since 25mm can be non insulated re BS7671 compliance).
PILC will someday require a phased replacement, something DNOs tend not to think about because nationally it is much more costly than water/gas where a new pipe can be pushed through the old one. Many PILC have been in service 80yrs and are still fine, so I guess it will not be an issue before 2020-2040 for the earliest installed cables. Comes down to soil thermal conductivity, current loading & other mitigating factors. I expect loop-in supply (2 properties off 1 cable) may be the first to show failure, because of electric shower in each property & electric cooker.
With underground distribution faults (PD, Partial Discharge), they boost the current briefly (called thumping) to detect where to effect cable repairs (by standing over the area and it goes thump). Those PILC cables are monitoring whilst the cables are online and likewise come under the maintenance strategy. We have a lot of old infrastructure in the UK, the (super)grid is old, the generating plant is a bit lacking with oil prices likely to push people from oil-boilers to electric-boilers, and overall it was designed in the 1950s & u/g thro the 1970s.