There are a few limits which apply to towing, technical, legal and advisory.
Technically the manufacturer will specify a limit for the vehicle and this must never be exceeded. This will be in the handbook, and in this case is 1800 kg for a braked trailer or 750 kg for an unbraked one. This is the maximum weight Skoda rate the vehicle to tow under any circumstances.
Legally there are different limits depending on the driving licence you hold. UK licence categories are here and it comes down to what licence you have and when you got it. Before getting into licence particulars, some terminology:
DGVM = towing vehicle's Design Gross Vehicle Mass i.e. maximum plated weight for the car. For the Superb Combi CR170 with DSG this is 2157 kg (check the plate on the driver's side B-pillar on your car to be sure, this is what's on mine)
This can also be referred to as MAM = Maximum Allowable Mass
MTPLM = trailer/caravan's Maximum Technically Permissable Laden Mass i.e. the maximum plated weight of the trailer. You will need to know this for whatever you are towing.
Most legal towing limits concern the combination of these two, often known as maximum train weight or combination weight.
All legal limits are subject to manufacturer's technical limits: the lower of the two applies.
If you don't have a BE licence (i.e. only a B licence), there are two scenarios: whether you passed your test before 1st January 1997 or after.
If it's before, you can tow any combination weight up to 8250 kg
If it's after, you can only tow up to a combination weight of 3500 kg, meaning you're limited to a trailer MTPLM of 3500 - 2157 = 1343 kg
Irrespective of vehicle DGVM, you can also tow a trailer with an MTPLM of 750 kg or less
If you do have a BE licence, there are also two scenarios: whether you passed your test before 19th January 2013 or after:
If it's before, you can tow any trailer regardless of its MTPLM provided the towing vehicle DGVM is 3500 kg or less (which it should be for category B anyway)
If it's after, the trailer MTPLM must be 3500 kg or less
For any Irish readers who see this thread, our rules are a bit simpler (and the same comment on legal vs technical limits applies):
If you only hold a B-licence, you can only tow up to a combination weight of 3500 kg, meaning you're limited to a trailer MTPLM of 3500 - 2157 = 1343 kg
Irrespective of vehicle DGVM, you can also tow a trailer with an MTPLM of 750 kg or less
If you hold a BE licence, you can tow any trailer with an MTPLM of 3500 kg or less
The final limit, which is not really a limit so much as a rule of thumb is the 85 % rule you've mentioned. As others have commented, this is a guideline for drivers with less towing experience to avoid running into difficulties when towing, especially with high-sided trailers like caravans or horse-boxes. It is based on the vehicle's kerb weight and this is going to vary depending on model and load. The figure on the V5 is the minimum kerb weight, and realistically the car is a good deal heavier (my car clocks in around 1700 kg with a driver and full tank of fuel). Based on the 85 % rule, and the V5 kerb weight, the OP has calculated 1354 kg, which is very close to the post-1997 B licence limit of 1343 kg, so isn't a bad figure here.
Something that will help with towing on the Superb is getting OE or equivalent towing electrics (Westphalia is the only one I'm aware of but there may be others). These integrate fully with other vehicle systems and enable some useful features for towing. The most useful on the road is the ESP towing mode which can use the ESP system to detect and help correct sway. The engine also detects when a trailer is detected and changes the mapping slightly to help with the load. Beyond that you get deactivation of the rear parking sensors and alarm activation if the trailer is unplugged while the alarm is set.