The most important things in a case about registered designs are:
i) The registered design;
ii) The accused object;
iii) The prior art.
And the most important thing about each of these is what they look like.
A key question will be does the design have individual character or what is the overall impression produced on an informed user is will be dependent on the arguments.
Overall it should be possible to decide a registered design case in a few hours.
You need to produce evidence from the designer, e.g. as to whether he/she was trying to make, or thought he/she had made. The evidence of experts particularly about consumer products, is unlikely to be of much assistance: anyone can point out similarities and differences, though an educated eye can sometimes help a bit. Sometimes there may be a piece of technical evidence which is relevant – e.g. that design freedom is limited by certain constraints. But even so, that is usually more or less self-evident and certainly unlikely to be controversial to the point of a need for cross-examination still less substantial cross-examination.
Most registered designs are for consumer articles, objects bought or to be appreciated by ordinary members of the public.
Do I need an expert. No. Not so for registered designs. Evidence of a technical or factual matters, as opposed to consumer ‘eye appeal’ may, on the other hand, sometimes have a part to play–that would be to give the court information or understanding which it could not provide itself.
In registered design actions, the court will be relucant to allow expert evidence.