Chrisi - es ist nicht klar, aus welchem Grund du eine 100% treffende syntaktische Analyse vornehmen willst. Hat es mit der Interpunktion (punctuation) zu tun? Oder sind es eher akademische/linguistische Interessen und Fragen, bis zu welchem Komplexitätsgrad ein Satz als 'simple' gilt?

Je nachdem würden mögliche Lösungen unterschiedlich ausfallen. Also bitte noch etwas mehr zum Kontext oder deiner Motivation, und warmherzige Zeitgenossen könnten vielleicht helfen.

chrisi: here's how to start:
1. Find and identify the subject
2. Find and identify the main verb
3. Find the direct object (if the verb is transitive)

So, how to find the subject and predicate? One way is to start throwing words out that seem unnecessary, not really unnecessary of course, but not central to the sentence, and keep chopping words out, until you have nothing left but the essential.

So, in your example, I would throw out 'In his early days as a concern-party artist for Combined Services' because it's obviously only a modifier and not central. Then, I would throw out 'both male and female' also obviously not the subject and not the verb.

So what's left? We now have: 'he specialised in imitation of voices'.

We can still throw out the prepositional phrase 'in imitation of voices', leaving:

'he specialised'

This is the central part of the sentence, and we now have:

Subject = 'he'
Verb = 'specialised'.

Since 'specialised' is intransitive, there is no direct object, and therefore this is a complete, grammatical, English sentence.

Now start with your basic sentence, 'He specialised' and start adding pieces of the sentence back in again, noticing their function in the sentence. For example:

'He specialised...' in what? '...in imitation of voices'. This is a prepositional phrase, led by the preposition 'in'. What is its function in the sentence? It tells you WHERE or HOW he specialised, so it MODIFIES specialises. Anything that modifies a verb is an adverb, therefore, '...in imitation of voices' is an adverbial prepositional phrase modifying the main verb.
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Now, keep going this way. What about 'In his early days as a concern-party artist for Combined Services'--what function does this have?

I won't answer this for you--you tell me the answer. When you think about its function, you will know the answer. Continue this process until you have added back all the parts of the sentence that you have stripped away, and you will have completed the analysis.

You may have noticed that the procedure is recursive--that is, the long phrases you strip out and add back can also be subjected to the same analysis, if you need to take it to that level.

But the main point bears repeating: find the subject and main verb (and D.O. if any) and start there.
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