Despite any opinions of safety it seems that gas tanks are manufactured to not accept more than 95% of their total volume because of regulations. Here is a quote from the US Department of Transportation Federal Motor Carry Safety Administration Regulation 393.67 Subpart E.
(12) Overfill restriction. A liquid fuel tank manufactured on or after January 1, 1973, must be designed and constructed so that—
(i) The tank cannot be filled, in a normal filling operation, with a quantity of fuel that exceeds 95 percent of the tank's liquid capacity; and
(ii) When the tank is filled, normal expansion of the fuel will not cause fuel spillage.
Chevron's Diesel Fuels Technical Review - page 34 says:
THERMAL EXPANSION Like all liquids, diesel fuel expands slightly in
volume as its temperature increases. The coefficient of thermal
expansion measures the rate of the expansion. A typical value of the
coefficient of thermal expansion for diesel fuel is 0.00083 per degree
Celsius (0.00046 per degree Fahrenheit). Using this value, 1.000
gallon of diesel fuel at
- 7 °C (2 0°F) will expand to 1.037 gallons at 3 8°C (100°F).
... meaning if your tank has a capacity rated at 25 gallons it is possible for it to expand up to 25.925 gallons in the described scenario. Meanwhile, a tank designed to hold 25 gallons of fuel must be designed to be at 95% capacity maximum, which means a tank that can hold 25 gallons will actually be a 26.31579 gallon tank to allow 5% for expansion. That means that in the above scenario there would be 0.39079 gallons of empty space left after expansion.
Though that scenario seems extreme, it is possible to encounter even more extreme situations even if it is unlikely. I wouldn't take the chance of overfilling the tank since it sounds dangerous and modifications described on the website you linked to might even be illegal.
I couldn't find official regulation pertaining to the UK but, there is a guide at gov.uk that says:
Diesel spillage is dangerous to other road users, particularly
motorcyclists, as it will significantly reduce the level of grip
between the tyres and road surface. Double-check for fuel leaks and
make sure that - you do not overfill your fuel tank
Whether or not it is legal to overfill a fuel tank it is still dangerous in practice.
I was curious and did a little more research on the differences of thermal expansion between gasoline and diesel fuel. I found the coefficient of thermal expansion for gasoline on answers.yahoo.com and wikipedia.org.
coefficient .00083 .00095
Same -7 Deg. C (20 Deg. F) to 38 Deg. C (100 Deg. F) temperature increase as above.
1 Gallon 1.03735 G 1.04275 G Difference of 0.0054 G (~= 4 tsp.)
Same 25 Gallon tank as above.
25 Gallons 25.93375 G 26.06875 G Difference of 0.135 G (~= 1/8 G)
% of total 3.735% Expansion 4.275% Expansion 0.54%
Which I take to mean that the author of the article you mentioned is technically correct that diesel expands less than gasoline but, he is very inaccurate describing diesel as expanding "much less" than gasoline. Looking at the comparison objectively, I would say that the thermal expansion of diesel and gasoline fuels are comparably similar rather than noticeably different. In fact, I would interpret the numbers as: "they both expanded one gallon in the 25 gallon tank."
I guess I'm trying to say that I don't fully trust the author of that article.